hero: allow hero diagonal movements

[jorgegv]

The hero cannot move diagonally, but it would be nice.

This would add 4 more directions for movement (besides UDLR), but sprites and animation sequences could be reused from the other directions (e.g. horizontal LR animations)

Tasks:

• Allow diagonal movement
• Modify collision detection for the new movement cases
• Fix bug in animation sequences:
  • Move right (press RIGHT), keep it and then move down (press DOWN) at the same time, then release DOWN, keep moving right. The animation sequence at this point should be RIGHT, but instead it is DOWN.
  • The same happens when moving LEFT
  • The same happens if you move UP while LEFT or RIGHT: sequences should be LEFT or RIGHT, but they are UP instead
  • The problem is reset if you start moving horizontally on the opposite direction

Enhancements to diagonal movevents are tracked in #80

[jorgegv]

Reopened for fixing diagonal animation sequences