wallpapers/image: Fix wallpaper thumbnails not adjusting to output size

Amends d6436e07.

This fixes wallpaper thumbnails not reloading after the screen size changes (it's likely a caching issue somewhere in the model, the relevant code is not easy to follow), it takes a load of complexity off the image model, and it paves a way for future simplifications in the model.

With the proposed approach, there is a dedicated image provider for wallpaper previews. The only thing that the model does is construct urls that are fed to Image. If the screen size changes, the qml code will change the Image.sourceSize property and the Image will do the rest for us.

Milestone: it's a pretty big change for 6.4, but it fixes a somewhat mild issue. On my setup, only a small portion of a kpackage wallpaper is visible because the primary output has portrait orientation so the thumbnail has to be scaled quite a lot to match the width of the output with landscape orientation.