kwineffects: Add QuickSceneEffect type

This change moves some pieces of the Overview effect in a new
QuickSceneEffect class that can be useful for other QtQuick based
effects.

The QuickSceneEffect class is meant to deal with rendering bits, input
handling, etc.

As the user of the QuickSceneEffect class, you need to override the
createView() method. The createView() method is invoked by the
QuickSceneEffect when it wants to create a QtQuick scene for the given
screen.

The design of the QuickSceneEffect is based on the Overview effect.