Background preflet initial version

Long live Background preflet.

Issue: #14
Change ToolBar accordingly to the contents

When we are showing the preflets grid,toolbar only shows a search box,
but when we are shoing a preflet the search box disappears, a back
action and the preflet title are shown instead.

Issue: #13
Hide the search box for now

Don't show it until we have a searchable categorized grid view.

Issue: #13
Custom style for group boxes

Make sure group boxes are styled the way we want despite the current
QtQuick Controls style.

Issue: #13
Remove "Other" category

This category is useless.

Issue: #13
Use non-symbolic icon for back action

Symbolic icons are so mobile.

Issue: #13
Show category icon alongside title

Improve appearance.
Expand toolbar height

Set ToolBar height to the tallest component.

Issue: #13
Don't link to VibeWidgets

We don't need to link to it anymore.

Issue: #13
Wallpapers model

Long live wallpapers model.

Issue: #14
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