Added world related events (tree_grow, world_load, world_unload, world_save)

It's like you want for the defaults. I added it because the AbstractEvent class with other default methods already exists and that I was thinking that adding others could a little more lighten the code yet.
The defaults only concern the second commit, so I can easily partially or totally revert it, if you or LadyCailin think that it's not a good idea.