Summary:
select the correct index upon loading when the count changes.
this removes the flicker at startup and the view is at the correct
wallpaper since the first frame. Also, now when a wallpaper is
downloaded with knewstuff, the old wallpaper stays selected,
it doesn't get resetted to the first wallpaper anymore

Test Plan:
open wallpaer dialog, no more flicker
install a new wallpaper, the currently selected one stays selected

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3532