- WorksheetControlItem: special element, for better UX, while user interact with cell. Now, this element handle drag-and-drop and cell selection (details below)
  - Multiple cells selection: now user can select not one, but many sells via Ctrl+LeftClick. Selection also visualizate on control elements
  - Actions on selection (first version): now user can apply some actions on selected cells, for example evaulating, deleting, moving.

Differential Revision: https://phabricator.kde.org/D27094