Keep completion popup on the screen

Pop it upwards when it would otherwise go below the screen and shift it
to the left when it would otherwise exit the screen on the right.

src/plugins/texteditor/completionwidget.h

@@ -74,7 +74,7 @@ private slots:
     void completionActivated(const QModelIndex &index);
 
 private:
-    void updateSize();
+    void updatePositionAndSize(int startPos);
 
     QPointer<QFrame> m_popupFrame;
     bool m_blockFocusOut;