Allow track to be expanded in the Layers component

[IanMayo]

Users may wish to change the label shown for a point on a track.

We only have properties at the whole track level.

But, we can store labels in an array of nullable strings. So, there is one entry for each point in the track - and this can be null or the value to be used for the label.

Use dynamic tree updates to populate the array of point items when a track is expanded.