Prevent trees (and their leaves) from growing outside the island

[Poslovitch]

Is your feature request related to a problem? Please describe.
Similar to #511, trees can grow a bit outside of an island's protection range, therefore making its leaves and maybe logs unbreakable by the neighbours players.

Describe the solution you'd like
A WORLD_SETTING flag to prevent this.

[Poslovitch]

I just did some tests and it's working like a charm! Trees at the border won't grow "outside", and trees outside the border just won't grow.

And it's even working in the Nether!

Well, I'll just have to update the en-US locale and add some unit tests before pushing that to develop 🙂.

[Poslovitch]

Committed. I had some trouble doing some unit tests - I had to @Ignore. Maybe we'll find a way to fix it someday.