Implement diagonal moves and fix some movement-related issues 

* Fixes #161
* Fixes #160
* Closes #155
* Fixes facing-based non-continuous animations

Other notes:
* In vertical/horizontal move cycles, the character waits now a bit each time it reverses its direction.
* The move frequency is now more accurate, but it's still not frame-perfect.
* You can no longer walk through Ib's parents at the start of the game.