Generalize AStarChaser

[filipbartek]

An AStarChaser object always targets an object called pacman. Generalize AStarChasesr so that it supports other target sprites in a manner similar to Chaser (e.g. AStarChaser stype=avatar).

[filipbartek]

Also note that the class AStarNode used exclusively in AStarChaser uses food, nest and moving hard-coded sprite names.