Add new base class for thumbnail plugins
This is a replacement for ThumbCreator, ThumbDevicePixelRatioDependentCreator, and ThumbSequenceCreator
It derives from QObject so that it can be used together with KPluginFactory
Input and output for the create method is encapsuled into request and result objects
This simplifies the DPR handling. We always pass the DPR as well as the already scaled request size to the create function and assume the creator handles it properly
It also addresses some TODOs from the current classes
Edited by Nicolas Fella