Fix/Improve PhysicsEnginePlatformer Jumping System

[Cleptomania]

The PhysicsEnginePlatformer class has the beginnings of a jumping system. In the platformer tutorial we have previously largely handled the jumping logic within the game, with only the use of the can_jump function to tell us if the player is currently on a ground or a ladder.

There is some logic in place for multi-jumps, but it is currently not working(the jump checks don't take it into account at all), and it also needs some improvement, namely an interval/timer between multi-jumps.

This stuff all appears in intellisense when users are working on games, so it would be good to get it actually usable, because newer users may be spending a lot of time trying to figure out why something doesn't work.