Implemented keyboard driver function for modyfing its current state.

Keyboard:
*Added a few TODO's to remember what is missing and needs to be done.
*Added a two functions for analyzing the scan code and changing the state
of the modifier/lock keys.
*Added the current_key_code field to the state structure. It will hold
the currently (lastly) pressed key. The alternative is some kind of
buffer to hold the history of pressed keys.