Implement KeyFile-based desktop file support to support non-Application type desktop files. Fixes #6.

This commit implements support for desktop files which cannot be handled by DesktopAppInfo, which is Type=Directory/Link.

To do this, we use GLib.KeyFile to load the file from its path, then check for the Icon and Name. If we fail to get an Icon and use a generic fallback. If we're successful, we set an icon name rather than attempt to load the icon, which saves us code by leveraging functionality in our DesktopItem.

For the name, we will attempt to get the "Name" key otherwise we set it to the display name of the file.

During launch of an item, we'll now check if keyfile is set. If it's set, we'll attempt to get the URL. If we get the URL, which should be set for Directory and Link types, we'll launch using the default handler for it.

Implemented a set_icon_from_name function which will attempt to load the pixbuf for the icon name provided. Refactored pixbuf fetching from the set_icon function into its own set_icon_from_iconinfo in case we need it down the road.

More aggressively prevent hidden files from being added. Since in some scenarios we won't have the FileInfo yet or we're dealing with a weird scenario like gedit saving temporarily to a dot file before moving it, we're doing it based on the basename of the file.

Use display name instead of label_name for file tracking in the file_items, since label_name is more likely to change.

We will improve how we handle non-Application-type Desktop files in 1.0.2.

Ref: #6.

Fixes #2.

Minor corrections to README.md

UnifiedProps handles the ownership of various properties leveraged by Budgie Desktop View, FileItem, MountItem, and DesktopItem. This saves us from either having a bunch of unowned properties that we're needing to update everywhere, lots of copies, and the ballooning of constructor properties such as additional settings.

Added build, run and uninstall instructions

This commit implements a dedicated right-click menu for FileItems and introduces a new gsettings key called "terminal" since there is no mechanism to actually "detect" what the default Terminal is. By default this is set to gnome-terminal, however an end user can set it to basically anything, though we only support the following out-of-the-box:

- alacritty
- gnome-terminal
- konsole
- mate-terminal
- terminator
- tilix

I documented with fairly extensive detail the flags supported by each Terminal application that we "support" and provide a warning (if you're running the desktop view process directly or checking logs) when you're trying to use a Terminal that isn't supported. Open to adding more, just need more testing.

This "Open in Terminal" option also does **not** hold the process open. It's my judgement that such preference such be left up to both the application and the user, for example in GNOME Terminal there is an option under Preferences -> whatever your default Profile is -> "When command exits" that decides what to do.

This implementation also comes with the caveat that GNOME Terminal is special. It's no mystery, but in this case it won't successfully set the working directory if you have a custom command set to run instead of your shell, and if you have "Preserve working directory" set to anything besides "Always". Bare that in mind.

This commit eliminates the child_activated handler on the FlowBox children. MountItem has its own left-click handling as a button_release handler, with no actions on right click. Special directories use the FileItem right-click menu. For FileItems that are applications, we prevent the FileItem menu from showing. This may be changed in the future.

Currently block right click on Desktop Items, though we may do something special there, like for Bash scripts.

Corrected license headers.

This can happen on a fresh install or an install that simply has never had files deleted via the file manager before. We'll provide a warning notifying that it doesn't exist, attempt to create it, and provide a warning if we fail.

Added a null checks for the trash_item so we're not trying to perform operations if the item doesn't exist.

Add more visibility checks. Force window positioning setting on show to ensure it is on default monitor not where mouse necessarily is.

Nautilus only presents the empty banner for Trash when opened as trash:/// instead of the .local path. If we determine we're launching the directory with Nautilus, swap to using the trash:/// URI.

1. Default to Home and Trash being enabled so it's immediately obvious when you have desktop icons enabled with an empty directory.
2. Have a more stable sorting of items since it wasn't properly handling multiple cases like hyphens, various casings, etc. This is being done through a new file comparator that uses collate key for filename generation, then strcmp, which is more efficient.
3. Don't push our special items into the file_items hashtable.
4. Change how we're iterating over max / over-max items.
5. Handle a case where Mutter and/or Gdk is dumb and misreports geometry changes, resulting in miscalculated columns, rows, window size, position, etc.
6. Simplify file fetching since FileEnumerator has a get_child method.
7. Tweak sizing, reduce margin and set that as a const. Maybe adjustable in future. Eliminated the DesktopItem padding.
8. Fix a bug where special folders and mounts were being shown when they shouldn't be.
9. Asynchronously load Pixbufs so we get FileItems added faster.