Remove deprecated g_type_init
tests: fix test-widgets
docs: remove references to old documents
viewport: subclass MxWidget rather than MxBin

Avoid using MxBin to allow the ClutterActor child management functions
(such as clutter_actor_add_child) to work correctly.
Remove MxBin class

It is not possible to use the ClutterActor child management functions
with MxBin, since it is not possible for MxBin to determine if the actor
being added was internal to the subclass or not. Therefore MxBin is no
longer used by any of the widgets.
Fix some minor compiler warnings
Remove the use of ClutterTexture

ClutterTexture has been deprecated, so this patch removes API that exposes
ClutterTexture and replaces the use of ClutterTexture internally. In most
cases, ClutterTexture can be replaced by CoglTexture, which should also
improve performance.

This patch also removes the MxTextureFrame actor (which depended on
ClutterTexture) since it was not useful on its own and can be
implemented in other actors by using the mx_texture_frame_paint_texture
utility function.