shell: Introduce workspace manager interface

shell: Introduce workspace manager interface

The workspace manager interface purpose is to provide clients with
control and knowledge about the current workspace state. Initially only
one function and one event exists; moving a surface and state updated
event. A workspace is represented as an index in a 1 dimensional array.

A client keeps track of the state by being broadcasted events when the
state changes, currently limited to current workspace or number of
workspaces available.

A client can send an asynchronous request to the manager asking to move
a surface to workspace identified by an index. It is up to the shell to
actually move it.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>