CppEditor: Refactor CppEditorWidget::contextMenuEvent

No behavior change.

* Extract some functions
* Add some clarifying comments here and there
* Add QTC_CHECK()
* Remove QLatin1String
* foreach -> for
* Const-correctness

Change-Id: Ie6ccc987ef959c5295a4d3225aac0dbad144f91d
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>