gtkwindow: Use default size even if not resizable

gtkwindow: Use default size even if not resizable

If a window is not resizable (with gtk_window_set_resizable ()),
the size given with gtk_window_set_default_size() is ignored.

The solution to this would be to use gtk_widget_set_size_request() but
that's a GtkWidget API and therefore does not take into account the
client side decorations when in use with GtkWindow.

Refactor the code so that gtk_window_set_default_size() (which is a
GtkWindow API) gives the expected result on non-resizable windows as
well.

bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=762974