Remove unused code
EventThread: Mouse button IDs greater than 'SDL_BUTTON_X2' are possible
MRI-Binding: Reduce number of object allocations

Previously, wrapped instances of mkxp core classes were
stored as RData ivars inside the actual object. This turned
out to be pointless as RData objects themselves are perfectly
valid objects that can carry ivars and have parent classes.
Therefore, the RData objects are now exposed directly to
the user scripts, effectively halving the amount of object
allocations.