Rename it into HighlightScrollBarController.
Don't derive it anymore from QScrollBar.
Make it based on QObject and decorate
the existing instance of QAbstractScrollArea as needed.

Fix the highlight of the shared scrollbar of the SideBySideDiffEditor.
Both left and right diff editors have their own
HighlightScrollBarController and their own separate overlays, but both
overlays are created as children of the same right editor instance.

Synchronize also the cursor between left and right editors.
Make highlight current line working.

Make the overlay transparent for mouse events - this fixes
issues on macOS when scolling over invisible scrollbar.

Change-Id: Iab05c360173e09d8748658c59785da86438a7189
Reviewed-by: David Schulz <david.schulz@qt.io>