Cache and resize wallpapers from QML

Instead of resizing and caching thumbnails from C++ we do it from QML.
This with asynchronous loading makes the stack view transition faster.
Hope it is enough to make it usable on low end machines.

However being loaded asyncrhonously, wallpaper items don't show up all
at once. We might want to display a progress bar in the future.

Issue: #17
Cache and load asynchronously the wallpaper preview

Same as what I have done for wallpapers in the grid view.
Let's see if this helps low end machines.

Issue: #17