Installing Community Collections from Flathub

[Til7701]

• Declare an Extension Point as described here: https://docs.flatpak.org/en/latest/extension.html
• Read collections from the declared directory and display them in the collection selection page. Use ExpanderRow to group collections from the extension.
• Provide example extension to make it easier to developers to create them.
• Explain the options in the community collections explainer popover.

Example extensions from other projects:

• https://github.com/flathub/org.videolan.VLC.Plugin.bdj/blob/branch/3-23.08/org.videolan.VLC.Plugin.bdj.json
• https://github.com/flathub/org.freedesktop.LinuxAudio.Plugins.GuitarML