BaseCellDelegate: Use correct colorSet when selected
When item is set selected, it will change its colorSet to Selection as well, but it will not change back to normal on deselect. See https://invent.kde.org/frameworks/qqc2-desktop-style/-/blob/master/org.kde.desktop/TreeViewDelegate.qml?ref_type=heads#L41
This makes sure we use the correct colorset here.
Edited by Akseli Lahtinen