Core: Signal duplicated editors

FakeVim needs some notification even in cases where the
duplication is triggered outside the EditorManager as in
QmlDesigner's qml-editor.

Fixes: QTCREATORBUG-22344
Change-Id: Ia4950ff4b02d3f89779f03ff4c106a07ccff2106
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/diffeditor/diffeditor.cpp

@@ -286,6 +286,8 @@ Core::IEditor *DiffEditor::duplicate()
     QTC_ASSERT(view, view = editor->currentView());
     editor->setupView(view);
 
+    emit editorDuplicated(editor);
+
     return editor;
 }