Don't include scrollbars in client area size

Return the size of just the viewport when using QScrollArea.

Closes https://github.com/wxWidgets/wxWidgets/pull/1418

include/wx/qt/window.h

@@ -230,6 +230,11 @@ private:
     QScrollBar *m_horzScrollBar; // owned by m_qtWindow when allocated
     QScrollBar *m_vertScrollBar; // owned by m_qtWindow when allocated
 
+    // Return the viewport of m_qtContainer, if it's used, or just m_qtWindow.
+    //
+    // Always returns non-null pointer if the window has been already created.
+    QWidget *QtGetClientWidget() const;
+
     QScrollBar *QtGetScrollBar( int orientation ) const;
     QScrollBar *QtSetScrollBar( int orientation, QScrollBar *scrollBar=NULL );