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.

Issue: #14
Show overlay for Plasma wallpapers

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

Issue: #14
Cache more thumbnails

Increase cache buffer.

Issue: #14
Create thumbnails to improve performance

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

Issue: #14
Rename MiniatureFileNameRole to ThumbnailFileNameRole

Seems much more appropriate.

Issue: #14