Now the icons to add or delete catalogs are only shown when mouse hovering a
catalog item in the tree. This is convenient for quick creation of catalogs,
and doesn't require activating a catalog to edit it first.
Determining if a tree item is hovered isn't trivial actually. The UI tree-view
code has to find the matching tree-row button in the previous layout to do so,
since the new layout isn't calculated yet.
75fbf6f17e