GDK W32: Adjust to new popup surfaces

GDK W32: Adjust to new popup surfaces

1) Handle GDK_SURFACE_POPUP in RegisterGdkClass()
   (for now pretend it's the same as GDK_SURFACE_TOPLEVEL)

2) Remove useless code from GDK_SURFACE_TOPLEVEL case in _gdk_win32_display_create_surface()
   (now there's just GDK_SURFACE_TOPLEVEL there, no need for a type check)

3) Have a separate case for GDK_SURFACE_POPUP and ensure that
   it doesn't get WS_CHILDWINDOW (and neither should GDK_SURFACE_TEMP).