[Wallpaper Image Configuration] Set cfg_Image whenever delegate becomes current

The delete button changes the currentIndex to something else when deleting the current image but that didn't update cfg_Image.
Just listen to when we become current. Might also fix keyboard navigation (didn't test if that worked before but it certainly does now).

BUG: 388805

Differential Revision: https://phabricator.kde.org/D9818