Make current item in ThumbnailView visible

Summary:
Render a dark border over current item
Render items that are being mouse hovered with a different palette

This isn't solving any bugs but hopefully should clear up some confusion
with currently existing split handling of selection and current items

Reviewers: KDE Applications, rkflx, ngraham

Reviewed By: rkflx, ngraham

Subscribers: ngraham

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D9025