containments/desktop: Store Folder View configuration paths relative to desktop

The Folder View widget used to show .desktop files' filenames instead of desktop entries' Name properties when using a user-specified path.

How to test:

1. Create a folder below xdg-user-dir DESKTOP and drag and drop an entry from Kickoff to this folder
2. In the Desktop Settings, make sure Layout is set to Desktop
3. Open the Folder View widget's settings
4. Choose the Custom path option and select the folder created in step 1
5. Before, the URI would use the file:// protocol, which would result in the file names of the desktop files being shown in the Folder View widget
6. Now it automatically converts it to a desktop:// URI, which results in the application names being shown in the Folder View widget

before	after

This MR converts all file:// paths below xdg-user-dir DESKTOP to relative ones using the desktop:// scheme.

BUG: 458400

@hein @ngraham