Adds an Alt+F fullscreen toggle

[luponix]

Adds a Hotkey class to define new key combinations in.
The background of the Alt+F fullscreen toggle is that on some linux systems overload doesnt get the screen dimensions leading to it starting with an incorrect size and window type every time even when attempting to overrule it with unitys screenwidth and screenheight arguments

[CCraigen]

I haven't scanned the code here yet, but I would prefer this becomes Alt-Enter to match the existing (at least on Windows) Unity hotkey for toggling fullscreen. I've recently run into an instance in D3 where there are rather "helpful" program-reserved Alt combinations preventing me from changing weapons while sliding down or trichording using down, or the existing OL problem of being unable to check the scoreboard while doing the same otherwise getting alt-tabbed out. I'd like to avoid more easily-hittable accidental hotkeys if possible.