shell: Virtual workspaces

shell: Virtual workspaces

A workspace is a list of top level surfaces visible at a time. New
toplevel surfaces are added to the current workspace. Default
keybindings (modifier - Up, modifier - Down, modifier - F1 up to F6) are
used for navigating between workspaces. By default a single workspace is
created.

Surfaces of inactive workspaces have their outputs NULL:ed so that frame
callbacks gets queued instead of emitted. When workspace gets visible
again surface's outputs are assigned.

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