Fix frame client size in wxQT, thanks @seandepagnier

Compute frame size subtracting statusbar, and handle NULL status bar

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77914 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/qt/frame.h

@@ -52,6 +52,9 @@ public:
 
     virtual QAbstractScrollArea *QtGetScrollBarsContainer() const;
 
+protected:
+    virtual void DoGetClientSize(int *width, int *height) const;
+
 private:
 
     wxDECLARE_DYNAMIC_CLASS( wxFrame );