Showcase: Add GestureListenerDemo

[mickleness]

Add a new demo to the Showcase app that handles Mac touchpad gestures (via the GestureUtilities class).

I have a working rough draft, but it requires fiddling with modules and permissions. Specifically it requires:
--add-exports java.desktop/com.apple.eawt.event=ALL-UNNAMED

I have to get back to other work, but I want to circle back to this. Someday.

[mickleness]

I submitted what I have so far in a new branch.

Also here is the terminal command used to access the new GestureListener Demo:

java --add-exports java.desktop/com.apple.eawt.event=com.pump -jar /Users/jeremy/IdeaProjects/pumpernickel/release/jars/Pumpernickel.jar

... and here's how IntelliJ needs to be set up: