Optimise pathfinding performance

[andersfischernielsen]

The current pathfinding implementation runs in O(n^2) and should be optimised using a priority queue. This would make finding Song-to-Song transitions much faster (O(n log n)).

[andersfischernielsen]

As far as I see it, the distance variable of the current algorithm should be changed into a PQ to allow following the current shortest path.