The problem was that certain layouts keep their "world" state in their value,
which was thrown away and forgotten after ReleaseResources during toggle.
In particular, decorated layouts store some X11 handles in them and
allocate/deallocate it as appropriate. If any modification to their state is
ignored, they may try to deallocate already deallocated memory, which results
in a crash somewhere inside Xlib.
This patch makes Transformers reversible so that nothing is ever ignored. As a
side effect, layout transformers now do receive messages and messages for the
base layout do not need the undo/reapply cycle -- we just pass messages to the
current transformed layout and unapply the transformer when needed.
(This, however, doesn't mean that the base layout is not asked to release
resources on a transformer change -- we still need the transformer to release
its resources and there's no way to do this without asking the base layout as
well.)
