Editors: Delete context menu if editor widget is deleted

...in CppEditor, QmlJSEditor and BinEditor.

Addresses second crash reported at QTCREATORBUG-8775. The first one is
already fixed.

Task-number: QTCREATORBUG-8775

Change-Id: I11c1f030ebeb2b2cbec02503e8e225fc5600a101
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>