shell: add private surface struct

shell: add private surface struct

Add a pointer to wlsc_surface for shell-private data. This is a
temporary solution.

Add struct shell_surface, where you can add any shell-private data
members related to a wlsc_surface. The getter function takes care of
creating the private data if it does not exist yet.

Not used anywhere yet.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>