TextEditor: Fix build for Qt5.9

Do not use functions that are not supported with
the minimum supported Qt for building QC.
Partially reverts 963dc84cc5.

Change-Id: Ife03143a7cf5a8f428754040e7004efe42d70a8a
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/clangcodemodel/clangdiagnostictooltipwidget.cpp

@@ -386,10 +386,14 @@ private:
 
     static int widthLimit()
     {
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
         auto screen = QGuiApplication::screenAt(QCursor::pos());
         if (!screen)
             screen = QGuiApplication::primaryScreen();
         return screen->availableGeometry().width() / 2;
+#else
+        return QApplication::desktop()->availableGeometry(QCursor::pos()).width() / 2;
+#endif
     }
 
 private: