Backpack - Add general "insert" slot

[JaXt0r]

To ease backpack handling, let's add a slot below the backpack to "add" an item into the backpack. It will be auto-sorted and added to existing stacks where appropriate.

TODOs

• Add Slot where any item can be added to - Bottom of Backpack
• When something is put into, always re-render the currently visible view - if we're on the same category right now
• If an item is pulled out of backpack, re-render the backpacks open items (to ensure we never have an empty hole)
• Backpack -> Check on sort order of items in inventory to align with G1. #321
• Backpack -> Ghostify items when pulled out of backpack #322

Bugs

• When an Item is in hand, it shouldn't be rendered inside backpack. At least -x amount of items in hand.
• When items are put Hand -> Backpack, they're currently removed from inventory, rather than put into. So "Put into backpack" should be reactivated to "add" items for inventory.
• Also implement Hip and shoulder slot to add items to inventory again
• RightShoulder too
• When put item into backpack via LeftShoulder (invisible backpack)
• Warning when grabbing: Setting angular velocity of a kinematic body is not supported.
  • When grabbed via hand/remote
  • When put into backpack
• Sometimes items throw exceptions when put into backpack. It might be related to a race condition of Events?