Work toward turning GdkDisplayManager into a backend singleton

This commit hides the GdkDisplayManager instance and class structs,
adds vfuncs for listing displays, opening displays, and getting and
setting the default display. The X11 backend has a derived
GdkDisplayManagerX11.

The gdk_display_manager_get() function is responsible for deciding on
which of the compiled in backends to use. Currently, it consults the
GDK_BACKEND environment variable and falls back to x11.

gdk/win32/gdkmain-win32.c

@@ -188,15 +188,6 @@ gdk_display_beep (GdkDisplay *display)
   Beep(1000, 50);
 }
 
-void
-_gdk_windowing_exit (void)
-{
-  _gdk_win32_dnd_exit ();
-  CoUninitialize ();
-  DeleteDC (_gdk_display_hdc);
-  _gdk_display_hdc = NULL;
-}
-
 void
 gdk_error_trap_push (void)
 {