Fix app ID and move Flatpak manifest

This MR does a few changes that fixes most issues related to Flatpak:

• Change the app ID entirely to org.gnome.Meld
• Move the Flatpak manifest to build-aux directory - everybody in GNOME does it
• Add .Devel suffix, just like all the other GNOME apps
• Add git module that builds git from source, instead of copying a file

Thanks to these fixes, GNOME Builder can now successfully build Meld from main without running into problems.

Fixes #797 (closed)