When the currentIndex in the ImageViewer changed, updates the grid's currentIndex too

Summary: * Adds a new property view in the currentImage, that holds the current gridView so that it can change the gridView's currentIndex too

Reviewers: Koko, mart

Reviewed By: Koko, mart

Maniphest Tasks: T6420

Differential Revision: https://phabricator.kde.org/D6454