Re-enable hover for items in ConceptTree

[stefandesu]

To improve performance, commit 168b6bb disabled the "remote hover" effect for concepts in ConceptTree (meaning that if you hover a specific concept elsewhere, it will not be hovered in ConceptTree as well). The reason this caused performance issues was that when there was a large number of concepts in ConceptTree, each would refresh its "isHovered" computed property on every single change to the globally hovered concept.

To re-enable this effect, the feature could be activated only for concepts that are currently visible in ConceptTree. The check which concepts are actually visible might itself have performance issues, but it's worth a try. See the checkInView function on top of this file for an implementation that checks whether an element is currently visible. This would have to be used on every change in the scroll position.