gtkapplication.c has turned into a bit of an #ifdef mess over time, and
many of the current checks are incorrect.  As an example, if you build
Gtk for wayland, and exclude the X11 backend, much of the functionality
required by wayland (such as exporting menu models) will be disabled.

Solve that by introducing a backend mechanism to GtkApplication (named
GtkApplicationImpl) similar to the one in GApplication.  Add backends
for Wayland, X11 and Quartz, with X11 and Wayland sharing a common
'DBus' superclass.

                             GtkApplicationImpl
                                      |
                       /--------------+-------------------\
                       |                                  |
            GtkApplicationImplDBus              GtkApplicationImplQuartz
                       |
           /-----------+-----------------\
           |                             |
  GtkApplicationImplX11      GtkApplicationImplWayland

GtkApplicationImpl itself is essen...