AdwAboutWindow: Add new_from_appdata constructor
The adw_about_window_new_from_appdata
function creates an AboutWindow with data automatically filled in from AppStream app data.
Appdata files are found by passing the prefix from your build system. The function will automatically scan the proper directories inside the prefix. An example of getting the prefix can be seen in the Adwaita Demo
This requires libappstream
present. It has been added to the Flatpak manifest temporarily, but should be added to the GNOME Sdk before merging