Add the Testing Feature back from AdventureCraft

[Starz0r]

AdventureCraft had a testing feature that allowed you to use things like keys, and doors, and triggers, without it being saved to the map after you are done testing. This was nice, and I'd like to see this make a return.

[Longor1996]

This requires that a deep-copy (or a deep-diff) of the entire active world is made, including all blocks and entities, and that copy be restored once the playtest-mode is disabled.

Note: Tracking every single change that is done and reversing them might be overkill. Although there is a cinematic camera mod that can do just that... I wonder how they did it.

Another note: Should the playtest be global for all players, or local to only one player?

If its supposed to be local:

• The entire world would have to be copied into another dimension to make things work.
• Other players must be able to join a playtest (chat broadcast w/ dimensional teleport?).

If its global:

• Are all players forced into playtest mode or only the one that started it?
• What happens with the players that aren't playtesting?