Since 93ccf4f2

 the IDocument base class has mimeTypeChanged signal
to cater for non-text document. This introduced the following:
QMetaObject::indexOfSignal: signal mimeTypeChanged() from
TextEditor::ITextEditorDocument redefined in TextEditor::BaseTextDocument

Change-Id: I5267e47bf28df1787dd959ae56cf022aaa858b14
Reviewed-by: Eike Ziller <eike.ziller@digia.com>