Cleanup Arcade Mode score character icon stuff

Centralize loading character icon for arcade score.
Added support for models/players/heads/*/icon_default.ext
Don't call trap_R_RegisterShaderNoMip to get score character icon handle each frame in postgame menu.
Fix GrappleState
Merge remote-tracking branch 'spearmint/master'

Conflicts:
	code/game/ai_move.c