fix(developer): new file support for handle project 2.0

Fixes #10276.

Adding new files to a v2.0 project was unsupported. This requires a
little bit of a logic change to the two new File New dialogs:

1. The New File Details dialog does not allow changing from the
   SourcePath of the project.
2. The New File dialog does allow changing to any path. If you select a
   path outside the project path, it will open in a new window.