On duplication of an editor onMimeTypeChanged() of CppToolsEditorSupport
was not triggered, thus not completion provider was set.

Task-number: QTCREATORBUG-10284

Change-Id: Ia1da7f59b19e042f65b068a993f0d5c04b0778e0
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>