Add position labels to interval tool handles while dragging

[matthew-blackman]

Kelly F. and @catherinecarter mentioned that it would be nice to have position labels for the interval tool handles while dragging them. Let's implement this as a quality-of-life improvement for the interval tool. The labels do not need to be stateful or instrumented.

[catherinecarter]

I love it! There are a couple layout issues that will need to be addressed. Not sure how, but here's what I found:

• the left edge and the "no air resistance" overlap (1st image)
• the mean readout gets lost when the handles are close together (2nd image)

[matthew-blackman]

Thanks @catherinecarter! Should be fixed now. Feel free to close if all looks good.

[catherinecarter]

Looks great. Nice work. Closing.