Don't delay ksplash until the entire slideshow is loaded
Instead of loading the model and then showing the last shown image, we can show
it early so that we signal "wallpaper.loading = false" earlier.
Test Plan: Have massive slideshow, login
Reviewers: davidedmundson, broulik, Plasma
Reviewed By: davidedmundson, Plasma
Differential Revision: https://phabricator.kde.org/D27084