Support side-view games

[jorgegv]

Add support for platform games in side view (i.e. gravity and floor detection support).

Since obstacle detection is already implemented (it's in the basics), only gravity management needs to be done. The platforms are obstacles.

Notes:

• With the current obstacle detection code and the implementation of gravity, Mario-style platforms can already be handled (if Mario jumps below a brick, he does not pass through, but bounces down; but if he falls on a brick, he keeps on it)
• We can define a new obstacle type TT_PLATFORM that can be passed through from below, but not from above (if the hero hits it from below, it behaves like a DECORATION but if the hero falls on it, it behaves like an OBSTACLE)