Cache difference of buttons down now and in the past

[GlenDC]

Right now to check wether or not a button/key has just been pressed, we check wether it's currently down and not yet previously. This is not a huge problem, but it's certainly not the best way. So it is probably better just to compute the difference and cache it, when assigning the fresh list to our state.

A cached list can be added both for keys and buttons, replacing previousButtonsDown and making the IsButtonJustInSet and IsKeyCodeJustInSet redundant.

This will also make the Update Logic a lot simpler...