Slots synchronisation

[osipxd]

It should be possible to synchronize the slot's content between different inventories.

Use cases

• synchronise vanilla armor slots with armor slots from custom inventory
• synchronise slots with a hotbar for quick access

Thoughts

• It should be possible to synchronise any slots' content
• It should be possible to synchronise GUI slot with storage slot. This might be useful to create GUI slots displaying equipped items
• Synchronisation should always be two-way to prevent item dupes

Blocked by #9