Customize track lengths

[OpenBagTwo]

Resolves #30, #32, #45

[OpenBagTwo]

Hmm... looks like something went wrong with the server-side registration. Max-level villagers are offering placeholder disks for trade.

[OpenBagTwo]

Tested this on a server:

• set the number of tracks to 12
• set the default track length to 180 (three minutes)

Logged onto the server with a seven-track resourepack with track lengths set by hand.

• A maestro sold me a placeholder track
• Sticking a (non-placeholder) track in the jukebox, it abruptly stopped playing at precisely the 3-minute mark

[OpenBagTwo]

Hmm... not sure why the new commits aren't being picked up. I'm going to try an old trick.

[OpenBagTwo]

Good to know the close/reopen trick still works I guess...