Fixes #280 by adding two new methods to StackSet for cycling workspaces

[sminez]

Adding two new methods to StackSet: focus_next_workspace and focus_previous_workspace which behave as their name implies. The definition of "next/previous" is in terms of the semantics of the ordered_tags method, so currently in terms of the workspace id. If the semantics of ordered_tags changes in future then so will the behaviour of these methods.

Testing wise there is a simple unit test that the cycling methods focus the expected tag both with and without wrapping at either end of the workspace list, along with a quickcheck test that for arbitrary StackSets with at least two workspaces we always change the focused workspace when calling either method.