Show loading indicator when loading tracks

[aganders3]

We should probably show the loading indicator when loading tracks, especially as experimenting with tack lineage shows loading large selections can be somewhat long. Initially I tried to do this in #29 but with a naive approach the indicator would show when loading tracks, but disappear if you changed the curTime. The simple fix I can think of is to hold separate state for loadingPoints and loadingTracks but there might be a better way to do it.

We may also want to disable some controls while loading, but hopefully this is not necessary.

[andy-sweet]

I think was closed by #87 so I'm closing it. @ehoops-cz : feel free to re-open if that is not correct.