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.
This is a work in progress to stub out an application class. The
primary goal is to provide a mechanism for applications to export
GtkActions, and there is a standard "Quit" action.
This is based on GApplication.
Future work:
* Add a way to say "This is my application menubar", which gets
put into all toplevel windows on non-OS-X, and into the top
on OS X.
* Support session management.
* Support application settings.
https://bugzilla.gnome.org/show_bug.cgi?id=127958
