There is no need to allocate memory for these callbacks for each single
GUI element. Instead, a single, statically-allocated instance can be
shared among all GUI elements of a given type.
`gui` was tighly coupled to game logic, and could not be extended for
other purposes. Therefore, a generic GUI implementation, loosely
inspired by well-known GUI frameworks such as GTK, is now provided, with
the following properties:
- Does not depend on dynamic or static memory allocation, only automatic
(i.e., stack) memory allocation required.
- Portable among existing implementations.
- Simple to extend.
- Tiny memory footprint.
`gui` is now composed by GUI elements that can be chained to form a tree
structure. This is useful e.g.: to calculate X/Y coordinates for a given
GUI element given its parent(s).
This commit also refactors the older implementation, moving
game-specific logic into `player` and making use of the new component.
