Rely on gapplication tool to start the application on legacy environment

If the environment does not support DBusActivatable, use the
gapplication command to start the application.
This fixes the fact than running the bare executable would not
show any window (since it's meant to be a DBus service), and
removes the need for one level of sed substitution.
Add infrastructure to build RPMS

Applications should be shipped by upstream, not by distributions.
Ideally, that would be done with some form of bundle, but until
those are possible, RPMs are good too.

This is where the full namespacing shows its strenght: because
everything is "org.gnome.Weather.Application", it's impossible
to have file conflicts with distribution packages (except for
gnome-weather itself), and there is no need for downstream patches.