Allow custom characters to define their own states for standard player animations

Custom characters can now replace individual player animations by creating states named after the following pattern: S_SKIN_<skin name>_<animation>.

Example: if state S_SKIN_BIGTHECAT_WALK exists and a player using the skin bigthecat enters its walking animation, it will switch to this state instead of S_PLAY_WALK.

A few notes:

• player.panim takes this into account and will be set to the correct value
• The super/NiGHTs transformation animation is an absurd mess internally and cannot currently be overriden as a consequence
• The animation names are exactly the same as the S_PLAY_xxx states

This merge request is part of a long-term plan that aims to make sprite2 eventually completely useless by replacing it with a proper animation system.