[GSoC 2020] Increase Cantor Plugins Performance

Now all panels belongs to CantorShell instead of Cantor Part, so now there is only one instance of each Panel in Cantor.
The changing of Panels content during Worksheets changes will be done via new methods: saveState, restoreState.
1 job for master
Status Job ID Name Coverage
  Build
canceled #58450
linux kf5-qt5 qt5.12