Support inventory import from dimension/playerdata

[hupster]

Made one addition that may be useful especially for per dimension inventories: import inventories from player files. If the user has no inventory yet for the new gamemode or dimension, look for the player file that may exist in the world directory of that dimension.

This allows adding old Multiverse worlds to the server as dimensions, in which the players will still have their inventories. If mods have been removed, the missing items are just left out of the inventory, with a warning on the console.

Even player data files that uses the 1.7 format (numeric item IDs) can be imported. Remarks on this:

• If the world has last been opened in 1.7, it will probably need to be opened in a 1.8 version first, to do the conversion of the world data. Version 1.12 doesn't seem to want to do that anymore. But the conversion won't convert the player files if they don't log in in version 1.8, so that still must be handled.
• The assumption is made that the world was part of the same server. The item ID's must match.