Add a view for screens

ScreenView contains all the layers and other components that need to
be shown on each screen.

At the moment we handle just one screen, though.
Hot corners initial implementation

This is fairly limited at the moment. An image is shown when the mouse
enters the corner and the logic to trigger is implemented however we
don't have actions to trigger yet.
Switch workspace with top left and right hot corners
Remove debug controls for workspaces