GApplication: set prgname to appid for services

Since services are based on D-Bus activation and desktop files are
supposed to be named like the busname for DBusActivatable applications
and since gnome-shell wants wmclass equal to the desktop file name, we
therefore want wmclass equal to the application ID in this case.

wmclass is determined from the prgname, which is otherwise pretty
pointless to set to some random thing in $(libexec) for a D-Bus service,
so set that to the appid.

This means that for D-Bus services, the following things are now all the
same:

 - application ID
 - prgname
 - wmclass property set on all windows
 - desktop file name
 - well-known bus name

There are not many applications running as D-Bus services at present so
this shouldn't impact anybody except for gnome-clocks (where this change
will be fixing a bug) and gnome-terminal.

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