[UI request] Rename `BBox Circle` slider and make it visible all the time

The `BBox Circle` slider was created to fix #1098, because the blue circles were sometimes too big. I've just discovered that the slider also affects the circles that appear around a selected staff, which is great for when images are small! But the slider only appears when the bounding boxes are visible, and the name of the slider doesn't make it clear that the staves will be affected too.

It'd be great if the slider was renamed `Blue circle size` (or something more eloquent) and visible all the time, not just when bounding boxes are selected.

This request applies to both standalone Neon and Mothra Neon.