Sub-classes of GtkApplication might want to override the way an
application window is created - for instance, to hook into GtkBuilder
or to set up some basic UI or state.

A new GtkApplication::create_window() virtual function is added to the
GtkApplicationClass vtable, which returns the newly created GtkWindow.
The gtk_application_create_window() function calls the vfunc and adds
the returned window to the list of windows managed by the application
instance.

Calling gtk_application_add_window() will also set the default window,
if one is not already set.

This commit also removes a spurious g_object_ref_sink() on the newly
created GtkWindow.