shell: Reorder events and have destructor for new interface

The new event should be last, and managed_surface should
have a destructor request where we free the resource.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>
shell: add title arg in "add_managed_surface" event

We now send the title at creation, too, for applications
that set it before showing their surfaces (simple-shm e.g.)

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>
compositor: hide the taskbar when locked

Hide the taskbar layer when locked, show it again after
resuming.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>