Add object-picking UI for inspector assignments

[kpreid]

It should be possible to change object references from within the inspector.

Prerequisite: design a UI for choosing the replacement object. Possibilities: popup menu (what contents?); drag-and-drop existing object chip onto new target; click replace button then click chosen replacement; etc. There should be a way to specify null, and it should be constrained to only valid types.

Once this feature exists, allow using it to replace:

• the blockset of a world
• a block type in a blockset
• a world in a block type