Better layer panel indication of droppability

When dragging and dropping layers, now only those layers get a full highlight that can actually accept the dragged layer as child (e.g. a group layer accepting a paint layer). Layers not able to accept the dragged layer as child won't be highlighted anymore and will only receive the insertion mark above or below. Not only can users can clearly see where the drop happens, but impossible "into" drops no longer result in a noop.

Differential Revision: https://phabricator.kde.org/D8057