libtiled-java: Do not store firstGid in each TileSet

Instead, they should be calculated on-the-fly when writing a TMX file.
Loading TMX files should parse the gids while the file is being loaded,
but not persist them inside the TileSet.
Merge branch 'master' into cache_tilesets

Conflicts:
	util/java/libtiled-java/src/tiled/io/TMXMapReader.java