Whiteboard backend polishing

by ÉJLS for Battle For Wesnoth

My project is to make the Whiteboard code cleaner and to redesign small parts of it to speed it up. The global design of the Whiteboard won't be changed a lot, each part will be reviewed individually. I'm not only planning to improve the Whiteboard backend, but also to document the overall design and each part of it as well as to write a wide variety of test to improve its stability. Moreover, I'll factorise action handling outside of the Whiteboard so that the same code will be usable in all Wesnoth.