CM:TS - PC build improvements

I made many improvements to the build system. You can now right-click to automatically slot items in appropriate slots. You can also click an empty slot to switch to the appropriate shop section.

I also spent some time to design universal input handling system, where every possible mouse/keyboard action is displayed at the bottom of the screen all the time. It's very prominent now, but I fear players might ignore it when the screen gets too busy with other stuff.