Rewrite the GUI code

[arthurmco]

• Make the GUI code better structured: As of now, it is highly hierarchied, and this is not good. Make something component instead of inheritance-based.
• Make it prettier: Text is not centered or aligned properly, buttons are ugly,,, The GUI needs to be prettier for a videogame, after all, this is a game, not an 1990's X11 application.
• Customizable: In the future, allow the developer and some future modder to define the control appearance in SVG.

We will continue to use Cairo to render the control, though.

[arthurmco]

The GUI code is better structured, and we can customize a lot of things, in a better way, using a common struct that the base control uses, instead of per control

SVG is not a priority now, so we can close this