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:

src/plugins/debugger/debuggertooltipmanager.cpp

@@ -624,11 +624,15 @@ void DebuggerToolTipWidget::computeSize()
     // Add a bit of space to account for tooltip border, and not
     // touch the border of the screen.
     QPoint pos(x(), y());
-    QTC_ASSERT(QApplication::desktop(), return);
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
     auto screen = QGuiApplication::screenAt(pos);
     if (!screen)
         screen = QGuiApplication::primaryScreen();
     QRect desktopRect = screen->availableGeometry();
+#else
+    QTC_ASSERT(QApplication::desktop(), return);
+    QRect desktopRect = QApplication::desktop()->availableGeometry();
+#endif
     const int maxWidth = desktopRect.right() - pos.x() - 5 - 5;
     const int maxHeight = desktopRect.bottom() - pos.y() - 5 - 5;
 

src/plugins/texteditor/texteditor.cpp

@@ -8252,7 +8252,11 @@ void TextEditorWidgetPrivate::updateTabStops()
     // to be set as an int. A work around is to access directly the QTextOption.
     qreal charWidth = QFontMetricsF(q->font()).width(QLatin1Char(' '));
     QTextOption option = q->document()->defaultTextOption();
+#if QT_VERSION >= QT_VERSION_CHECK(5, 10, 0)
     option.setTabStopDistance(charWidth * m_document->tabSettings().m_tabSize);
+#else
+    option.setTabStop(charWidth * m_document->tabSettings().m_tabSize);
+#endif
     q->document()->setDefaultTextOption(option);
 }