Since 3b598b6f75 TextArea comes with a built-in
context menu. Here we demonstrate how to add extra menu items to the end.
Pick-to: 6.9
Task-number: QTBUG-35598
Task-number: QTBUG-134903
Change-Id: I1e3d8c044939521aaa076486630e3b879130dfd1
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>