Display and edit tileset names

It's now possible to view and edit the name for every tileset. This will enhance the clarity for the user if he can assign his own name for a tileset. This will also be done the same way for all future elements (Maps, sprites,...).
The tileset names are stored in and loaded from a xml file. It will be possible to support different languages just by changing the xml file.

Another small progress:
Tilesets 1, 2 and 14 (can be seen in the screenshot) are now displayed correctly. The issue was a wrong byte offset where I started to read the tilesets.