Window shortcut

I've added all of window_shortcut and its sub window.

I've also added the ability to read the existing keyboard assignment. This doesn't work quite right as openrct2 language.txt does not layout its keys in the same order as sdl2. I could write a function that rejigs the keycode into a openrct2 keycode or we could just reorder language.txt files. Its not a huge issue for right now. 

Fixes #198.