- Handle mouse events (drag and resize for floating windows)
  - Eventually click to WS switch and other actions from the status bar but that's a long way off

- more hooks
- more tests!
- Selector based methods that return all matching items rather than just the first
- re-order the methods on WindowManager
- Fix auto-float behaviour
- 'tags' extension
- focus next (condition) action
- focus_or_spawn action
- move client to screen built-in action
