This is another step in the gobjectification of the internal library
of gstreamer-vaapi. Now it is the turn of GstVaapiWindow and its
derivates.

The idea is to minimize the changeset keeping the same design as
much as possible.

GstVaapiWindow is defined as an abstract class with two properties:
the GstVaapiDisplay and the native ID. Thus, many of the
GstVaapiObject macros were copied as GstVaapiWindow macros.

The function gst_vaapi_window_new_internal() is kept as a decorator
of for calling gst_vaapi_window_create() and the possibility of
failure.

The descendant classes, such as glx, still use the private
structures, but through the gobject mechanism.