Window Thumbnail support for QML

A new QML item "ThumbnailItem" is registered to the TabBox. The
C++ implementation finds the EffectWindow of the TabBox and adds
itself to the EffectWindow.

While rendering the EffectWindow the information for all registered
ThumbnailItems are extracted and the thumbnail is rendered on top
of the EffectWindow.

This has obvious limitations like you cannot put other QML items
on top of the thumbnail. Nevertheless it works well enough to
be a possible replacement for e.g. BoxSwitch effect.

When compositing is disabled an icon is rendered instead of the
Thumbnail.

One TabBox Layout inspired by BoxSwitch Effect is added. For the
KCM small pre-rendered items are used.

REVIEW: 103039