Making the selectionHighlight visiblity status dependent on the m_selectionModel of the Koko.SortModel

Summary:

• Highlighting of the items now depends on whether the item is selected or not in the m_selectionModel
• Adds a new SelectionRole to SortModel

Reviewers: Koko, mart

Reviewed By: Koko, mart

Subscribers: Koko

Differential Revision: https://phabricator.kde.org/D6393