A* algorithm visualization

version 1.3
- small fix concerning the binary heap

version 1.2
- now three different heuristics to choose from
- main menu overhaul
- key now hides properly
- changed closed list to save memory
- updated binary heap
- buttons no longer get partly cut off on large maps

version 1.1
- changed the internal data structure of the open list to a binary heap
- algortihm now displays a message if no path can be found