GUI elements

I've added some functions (some were already available on Pine Client). As well as CSS to style the interface.

Documentation for the GUI: https://github.com/Open-RPG/Pine-Client/wiki/GUI

The goal of this class is to define inside it all functions that affect the interface. By interface I mean the elements that are not a part of the game, so game characters, objects, houses would not be a part of the GUI.

A more general approach to know when to declare things inside GUI is:

* If it's drawn on the canvas: *Not* GUI
* If it's an HTML overlay: GUI