Added option to use mouse pressure to show the dock.

FEATURE BRANCH - experimental_pressure

Hi Michele,

I've added a pressure sensitivity feature that incorporates GS 3.8 pressure barriers to sense for mouse pressure against the left edge of the screen (right edge in RTL) the dock is positioned on. This option not only helps eliminate accidentally triggering the dock, but also makes the dock more accessible in dual monitor configurations where the secondary monitor is to the left of the dock .. especially when intellihide is turned off. Of course, this feature requires Gnome Shell 3.8+ and an XServer installation that implements pressure barriers.

NOTE:
In such cases where the secondary monitor is to the left of the dock, the dock will have to be showing before the pressure barrier is removed and the mouse pointer released to access the secondary monitor. This will create a slight hesitation as the mouse must wait for the dock to show. To elliminate this issue, you may show the dock by entering Gnome Shell's overview mode prior to attempting to access the secondary monitor.

While I plan to do more thorough testing, I've sent a pull request because I thought you might be interested in looking through the code. Let me know if you think this feature would be useful.