GApplication: implement fd.o application spec

The freedesktop application specification is largely overlapping the
GLib application D-Bus interface but implementing it will allow for
applications to be launched directly from desktop files, which we want.

We keep the old Gtk interface for compatibility reasons and because it
has some functionality not in the freedesktop spec (Busy state,
CommandLine, etc.).

https://bugzilla.gnome.org/show_bug.cgi?id=699259