Backhand

### **Backhand v.1.3.0**

New features:
- The offhand item is now stored in a player extended property when not assigned to an inventory slot, meaning many more mods that change the player's inventory are compatible with fewer conflicts.
- A similar change to how the offhand slot container is implemented now allows more mods that change the player's inventory container to be compatible with fewer conflicts.
- Most items are mirrored correctly in first and third person in the offhand now.
- More items like fishing rods and thaumcraft wands are now usable through an added mixin and ticking server-side hotswap. The ticking hotswap is toggleable through the config.

Fixes:
- Fixed ghost blocks when breaking blocks with the offhand in creative mode.
- Fixed attacking an entity with the offhand right after using an item or interacting with an entity with an item.
- Fixed server-side crash when checking for offhand use and shift placing offhand blocks on blocks with uses.
- Fixed null pointer exception in offhand creative inventory.
- NetHandlerPlayClient.handleSetSlot() mixin now sets item stacks to null in slots if their stack size is zero, fixing some container issues.
- Fixed nullpointerexception client-side console spam on sent packets.
- Server-sided configs are now synced with the client.