Merge branch 'TimePath/itemsys' into 'master'

TimePath/itemsys: an item and inventory system

This system and the weapon system seem to overlap quite a bit and would benefit from being combined, currently this one ignores weapons completely.

Current state:
 * All items except weapons are spawned using this system
 * Tracks item pickups from clients and networks their inventories to them
 * Items can define their own behaviour to actions without conditional logic throughout the code

See merge request !128