Added a few more generic parts to the input library:
Created Mouse and Keyboard to join JoyStick and provided them with a templated base class (InputDeviceTemplated) that does most of the work (reduces quite some redundancy)
Created InputPrereqs.h from InputInterfaces.h and destroyed the latter
Exported InputHandler to its own file and replaced KeyHandler, MouseHandler and JoyStickHandler with the single InputHandler.
Deleted the SimpleInputState: There is only one class now which fulfills all our needs.
In general there is now less code and the code itself has more 'pluses'. However I haven't really thrown away any feature at all.