Allow adding and removing tilesets to existing maps

[Brett208]

• Operation is aborted if tileset is contained within a TerrainType.
• Removes references to tileset from TileGroups and the current map by setting indices to 0.

** This functionality will eventually need to be expanded for dealing with TerrainTypes.

[Brett208]

I resolved the merge conflicts within GitHub and it only added a single commit in the process, which is good. I found GitHub's merge conflict resolution system intuitive. The only downside is that I cannot compile and check on the results before pushing the merge to the public.

I will try to resolve through TortoiseGit for the next Merge Conflict and see how it goes again.

[Brett208]

I'm considering never merging this branch into master. A system that gracefully handles replacing tilesets needs to take into account the contents of TerrainRange, which may look a lot different than this solution.

I would like to leave this branch in place in case it is helpful to someone in the future.

[DanRStevens]

I can see where you're coming from with this. The code is generally useful as part of a solution to the problem, but it's not complete, and not really usable without being complete. Adding it in now adds a substantial amount of code to the project, without any real new useful functionality, though discarding the code also seems wrong, since a working solution will likely need most of this.

What about separating the removal from the compaction of the tileset indicies? That might segregate some of the ugly parts of this problem. It would also make sense if you were doing a replacement of part of the tileset, where an intermediate compaction step might be undesirable. Perhaps the user might want to do a pure graphical replacement, with 1-1 equivalences between tiles. Though considering that case might be an attempt at over optimizing.