Implement wxWindow::DoGetBorderSize() in wxUniv.

As some wxUniv classes implement DoGetBestClientSize(), the new code in
wxWindow::DoGetBestSize() implementation calls DoGetBorderSize() which asserts
because it's not implemented, making it impossible to even start the minimal
sample -- fix this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62002 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/univ/window.h

@@ -243,6 +243,9 @@ protected:
     virtual wxSize DoGetBestClientSize() const;
     virtual wxSize DoGetBestSize() const;
 
+    // override the base class method to return the size of the window borders
+    virtual wxSize DoGetBorderSize() const;
+
     // adjust the size of the window to take into account its borders
     wxSize AdjustSize(const wxSize& size) const;