Placing objects on the toolbar in conveyor testing mode

[Jennynot]

Issue: I love the idea of this, but it's really hard to switch between resources in the hot bar. I end up with loads of duplicate piles of resources because I can't seem to swap from eg belts to conveyors.

Suggestion: have the hand 'contents' be dependant which item is selected, and only have those two options available on the toolbar.