Extended tile set store

[tordanik]

This PR improves handling of extended tile sets. Notably, it encourages multiple map elements (nodes, ways, relations) to reference the same extended set by ...

• using the same indices, and refraining from modifying existing sets
• adding the "primary" tile to the extended set as well, thus allowing e.g. all nodes of a way to use the same set even if they are in different tiles themselves
• looking at the most recently added set for reuse potential.

A new ExtendedTileSetStore class handles adding and retrieving tile sets (growing the backing array as needed, much as an ArrayList).

This pull request also includes some cleanup work and is based on #32.