Prevent dock from accidentally activating and sliding in by testing for mouse hover.
Remove pressure barrier when an app goes fullscreen.

Note: the tracked slider actor is hidden by Main.layoutManager when an app is 
fullscreen.
Remove the pressure barrier when the messageTray is showing.
Initialize barrierTimeoutId and remove it from mainloop before destroying dock.
Don't remove pressure barrier if fullscreen app is on secondary monitor.

Fullscreen apps on the same monitor as the dash will cause the dash slider to 
be invisible.