[WindowThumbnailItem] Expose the client's frameGeometry as implicit size
Set the implicit size of the item to the client frameGeometry size.
This is also more convenient for tabbox usage than accessing it through the client's property, had a proper notify signal and abstracts it from the implementation.
It also can be mimicked by the preview ThumbnailItem used in the tabbox KCM, which has no client property.
Edited by Ismael Asensio