app: Build AboutWindow from AppStream data
Use libadwaita's Adw.AboutWindow.from_appdata()
constructor to build
the about window, filling in most details from the appstream data XML
file.
This change means we'll no longer have to keep the metadata in 2 separate places, and allows us to show the release updates from the about window as well.
Note that we have to do a bit of awkward version juggling as the AppStream specification does not deal with alphabetical components inside a version (so we use $VERSION~alpha instead of $VERSION.alpha).