Add a translucent white background for zoom labels
Scale zoom labels to fit within their associated item
Do not show a zoom label for RootItem
Remove the functionality to change an overlay's associated item(s)

This makes it easier to work with overlays
Add more functionality to overlay groups
Make zoom labels avoid overlap by hiding labels under top-level ones
Prepare zoom labels for a better way to handle overlaps
Implement a push-away strategy to avoid zoom label overlap

It is not the best solution, graphically, but it is very fast.