[Image Wallpaper Slideshow] Allow setting of different sorting orders
Allows setting of other sorting orders like alphabetical or last modified date.
To enable this a new ProxyModel is introduced which handles the sorting and
filtering (as indicated by the checkboxes). This is backed by the slideshowModel
whcih previously as only used for the configutation. The lists of slides and unseen
slides are dropped as now the slides that are shown are taken from the model.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, msdobrescu, ngraham, filipf, plasma-devel
Differential Revision: https://phabricator.kde.org/D22121