Add measurement tool w.r.t. a frame

[adamkewley]

Estimate: 0.5-1.5d

There's currently a hacky spatial measurement tool, but one thing that people are asking for is a measurement tool that measures things within a particular frame (coordinates, lengths, whatever).

The idea would be that a user selects some frame in the scene and then starts clicking somewhere else so that the UI says "oh yes, it's here in space and it's this far across, etc.)