Controller polish: GUI fixes, +/- axis fixes, etc

Tested and improved all the click, shift-click, click-drag, right-click
inventory features.

"button" bindings now bind to only either a + or - axis. If you have a
controller which has analog triggers which are 1 on fully depressed, but
-1 on fully retracted, you'll need to map bindings accordingly.
(Original Xbox controller, I'm looking at you).