New mod: Item Selector

This is an implementation of a mod idea I had last night. Basic function involves scrolling the mouse wheel whilst holding down a modifier key (default left ALT). This brings up an overlay of the player's inventory with a moving selection.

When the player pauses over that selection for a settable amount of time, the selected hotbar item gets swapped with the selected item on the overlay. However, if the player mouse scrolls whilst the overlay is open but without holding the modifier key, the selection is cancelled.

This is a basic implementation, complete with keybinding, config and GUI. May need to improve the visibility of the overlay, perhaps with a dark background. In future, this mod could be adapted to use a "favorites" menu instead, even perhaps per-slot and for creative.

## Example screenshots:
1. Before item selector is used: http://i.imgur.com/blzJp5U.png
2. During use for the second hotbar slot (mousewheel scroll + left ALT): http://i.imgur.com/jF4H1WF.png
3. After waiting the default 200 ticks, the item is swapped: http://i.imgur.com/y9FLWUs.png

## Other changes
This pull request has some commits which makes changes, both intentional and collateral from the use of IntelliJ. For instance, unusually the revision I forked off of would not compile until I made two fixes.