Base ShellStack on StContainer rather than ClutterGroup, so that it
has StWidget-y features (and so we don't have to "cheat" in
shell_stack_allocate()). Implement navigate_focus() to only ever pass
focus into the top-most child, since doing otherwise would be
surprising.
https://bugzilla.gnome.org/show_bug.cgi?id=646934
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.
