This enables adding the Typeable constraint to LayoutClass itself
(https://github.com/xmonad/xmonad/pull/242) which in turn enables
querying the current layout state. That might be useful to e.g. show the
current X.L.WorkspaceDir in xmobar.
This is a preparation commit that fixes the compile failures that would
result in merging that change to xmonad. For this to be generally useful
we first need to merge (and ideally also release) that xmonad change,
and then we'll need some documentation and perhaps a type class to help
find the right LayoutModifier in the tree of ModifiedLayouts and
Choices. That will come later.
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.)
