Rename highlighted elements to visible

Closes #625 (closed)

This removes the confusion between the ?highlight query parameter and the notion of elements being shown or hidden on the image by clicking on the in the children tree.

assigned to @erwanrouchet

changed the description

requested review from @babadie

added 3 commits

• 2fd3802e...0b2122de - 2 commits from branch master
• 6b8c86dc - Rename element highlight to visible

Compare with previous version

mentioned in commit 5d5eca87

merged