Add support for merging tilesets

[stefanbeller]

It would be great if we could merge tilesets (remove unused tiles from all tilesets, store the remaining tiles into a single tileset and apply that to the map).

This project aims at doing that (or something very similar): https://github.com/falanxia/tileset_baker/wiki

(creating this to close the issue at the old bugtracker https://sourceforge.net/apps/mantisbt/tiled/view.php?id=36 )