PageStack and StackView

[Kai Uwe Broulik]

I migrated the essential parts of PageStack to StackView but now System Tray 
is completely broken. Switching to different applets leaves the old ones 
there.

According to documentation "By default […] StackView will destroy items pushed 
as components or URLs. Items not destroyed will be re-parented back to the 
original parents they had before being pushed onto the stack and hidden." but 
since the taskItemExpanded is a QQuickItem* member of Task it cannot really 
re-parent it and just dumps it somewhere where it stays visible :/ 
DestroyOnPop did not work either and is probably not desirable anyway.