Look for QtXdg instead of Vibe

We don't need Vibe libraries anymore, all Desktop Entry stuff was moved
to QtXdg.
Load Plasma wallpapers

Plasma wallpapers has a desktop entry with metadata, a low resolution
representation of the wallpaper and several images for each resolution.
Show overlay for Plasma wallpapers

When the mouse is over a Plasma wallpaper thumbnail, show an overlay
with information.
Cache more thumbnails

Increase cache buffer.
Create thumbnails to improve performance

Scale down backgrounds and cache the generated thumbnail.
This will make the preflet faster and less memory hungry.