GameWorlds and GameModes

[Poslovitch]

See #408.

I just started the branch by creating an UML Class Diagram, which I'd like us to review before I start working on this - and so we are actually 100 % about what's going to change.

[tastybento]

I had a go at the implementation so please have a look at it and let me know what you think. It's okay to criticize! I'm not sure if I got it right.

A few comments:

• GameMode is already taken by Bukkit, so I used GameModeAddon, which is an accurate description
• The general approach is that when a GameModeAddon is loaded, BentoBox will automatically call the createWorlds() method that the GameModeAddon must override. This method must generate worlds and assign them to the appropriate variables. After that, BentoBox will then load any schems in the add-on jar or schem folder automatically. This is nice.
• I couldn't work out how GameWorld class would work. Maybe my brain is soaked with too much Christmas wine! Anyway, I just put the required methods into GameModeAddon because it seems to fit there. Please feel free to explain it more if you think it's needed.
• Have a look at the AcidIsland branch I made that uses this API. You can compare the differences. With this new approach, I get rid of a whole class and the code is generally nicer. Let me know what you think. Again, I may have gone completely in the wrong direction, so let me know.
• GameModeAddons no longer have to explicitly declare the world they apply to in a CompositeCommand because it can be obtained automatically. This is nice. There may be more optimization of the code possible now that GameModeAddons exist.

Merry Christmas!

[tastybento]

Easy comparison link: develop...gameworlds-and-gamemodes

[Poslovitch]

Okay, I cannot approve nor request changes as I'm the PR's author. But yet, I approve the merge of this PR.

[Poslovitch]

There is a conflict with SchemsManager. Does this PR supports the multiple-schems that were added recently?

[tastybento]

Let me resolve the conflicts.