Add ShellDrawingArea and ShellStack

Add ShellDrawingArea and ShellStack

ShellDrawingArea is a size-independent wrapper for a ClutterCairoTexture.
Useful when drawing non-fixed size areas.

ShellStack is a simple container class which holds items
in a completely overlapping Z stack.  The main difference
from ClutterGroup is that items will be constrained to
(and allocated) the size of the stack, not getting their
preferred size always.