Allow specifying all wxFlexGridSizer parameters in XRC.

Add support for specifying flexible direction, grow mode in non-flexible
direction and row/column proportions for the growable ones.

Closes #14767.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72710 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/xrc/xh_sizer.h

@@ -51,6 +51,7 @@ private:
     wxSizer*  Handle_wxWrapSizer();
 
     bool ValidateGridSizerChildren();
+    void SetFlexibleMode(wxFlexGridSizer* fsizer);
     void SetGrowables(wxFlexGridSizer* fsizer, const wxChar* param, bool rows);
     wxGBPosition GetGBPos(const wxString& param);
     wxGBSpan GetGBSpan(const wxString& param);