build: Update git.mk from upstream

https://bugzilla.gnome.org/show_bug.cgi?id=720853

Signed-off-by: Álvaro Peña <alvaropg@gmail.com>
build: Split EXTRA_DIST and CLEANFILES up in data/Makefile.am

Make each section of the Makefile self-contained.

https://bugzilla.gnome.org/show_bug.cgi?id=720853

Signed-off-by: Álvaro Peña <alvaropg@gmail.com>
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.

https://bugzilla.gnome.org/show_bug.cgi?id=720853

Signed-off-by: Álvaro Peña <alvaropg@gmail.com>
app-data: Updated screenshot dimensions to a 16:9