Replace manual signal blocking/unblocking with QSignalBlocker

Change-Id: Ibb59fab4e37d045e506c5a8172b6f5cbb955b028
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/scxmleditor/plugin_interface/textitem.cpp

@@ -84,9 +84,8 @@ void TextItem::checkText()
 
 void TextItem::setText(const QString &t)
 {
-    blockSignals(true);
+    QSignalBlocker blocker(this);
     setPlainText(t);
-    blockSignals(false);
 }
 
 void TextItem::focusInEvent(QFocusEvent *event)