We only care whether the background is stretched across multiple monitors
or not, let's be explicit and make it clear.

* Packaging: Fix Debian build

* Revert "CI: Don't build for LMDE3"

This reverts commit 04592162.

This adds the option "only-on-monitor".
Default is "auto", which means "Follow the mouse", like it was without
this option.
This goes together with linuxmint/lightdm-settings#17

Fix typo

This option required a settings daemon and is no longer used.

Also, add new options to README and man page.

* fixed README

Add a screenshot

When a user is added, check his/her session and make sure it's valid.
If it isn't, look for a known session name in /usr/share/xsessions,
and use that instead.