build: Enable validation of the AppData file

Add a build-time dependency on appdata-tools to enable validation of the
AppData file during `make check`. See:
https://github.com/hughsie/appdata-tools.

This also requires an up-to-date version of gnome-common to pull in the
appdata-xml.m4 macros.

https://bugzilla.gnome.org/show_bug.cgi?id=720854