WorldInfo will now save world dimensions to level.dat. Fixes #1128

When using world plugins such as MultiVerse, restarting a server or
unloading a MV world would cause dimensions to change after reload. To
fix this issue, all dimensions will now save its dimension ID to the
corresponding level.dat file in its world directory. For example, if you
create the world "terrain1" using MultiVerse, the world will be created in
overworld/terrain1 and the dimension will be saved to
overworld/terrain1/level.dat.

Note: Since this is a new feature regarding saving, please backup your
worlds before updating.