So far, their position inside the container was determined when
initializing the interface. However, if a child were resized
afterwards, the container would not adjust its elements accordingly.
Moreover, the implementation for gui_container relied on hacking the
children's X/Y coordinates, which could only be done once.
Now, two additional members have been added to gui_common so that
specific X/Y offset can be determined by the parent, additionally to the
traditional rules followed by gui_coords. Despite the extra memory
footprint, it now allows containers to set specific X/Y offsets for
their children on every game cycle.
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.
