QmlDesigner: Override shortcuts in text editor

There are a couple of known shortcuts that we have to override
in the text editor. We install an event filter and handle
the shortcut override event for the keys in question.

Change-Id: Icb5483b19e2202d85e99d24c20d05097aaa33da6
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/components/texteditor/texteditorwidget.h

@@ -58,6 +58,9 @@ public:
     void clearStatusBar();
 
     int currentLine() const;
+protected:
+    bool eventFilter(QObject *object, QEvent *event) override;
+
 private:
     void updateSelectionByCursorPosition();