Appstream support

[mcatanzaro]

It would be nice for the GNOME module to have an equivalent to the APPSTREAM_XML macro provided by appstream-glib:

https://github.com/hughsie/appstream-glib/blob/master/data/appstream-xml.m4

It looks like it doesn't do much:

• Installs the appdata file to the right place
• Validates it during distcheck (I guess that could be done at check time in meson)

[TingPing]

This is part of #837