Add more obvious UI feedback when data is loading

[dstansby]

I often load up a dataset in Neuroglancer, and then have to wait for a little while for the largest resolution image to load. I'm always a bit unsure if something is loading or not, so it would be good to have some clear visual feedback in the UI when data is loading.

[stuarteberg]

The UI does have a small progress bar for loading meshes. It's shown in the layer's settings panel, so if you've got meshes loading from multiple layers, I guess you'd have to check them all.

[dstansby]

Ah I actually meant loading volume data (not meshes). Thanks for pointing out the numbers, would be good to maybe have a little spinning icon next to the number to indicate something happening?

[jbms]

For volumetric data, the "resolution (slice)" control is the relevant one.

There is also a second loading indicator --- in the layer bar, the very top line of the layer tab shows the loading progress for visible data, and the bottom line shows the loading progress for prefetched data. However, these indicators are very subtle --- perhaps they should be made a bit more prominent.

In typical use of Neuroglancer data is being loaded much of the time, because the user will likely change the viewport to some extent even before everything finishes loading. For that reason I did not want to make the loading indicators too prominent, as they might become distracting.