Implement wxNotebook::CalcSizeFromPage() for wxGTK.

The implementation is far from perfect as it relies on hard-coded margins but
is better than nothing as it allows wxNotebook best size determination and
SetPageSize() method to work correctly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70112 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -459,6 +459,10 @@ All (GUI):
   wxDataViewCtrl (Andrew Xu).
 - Fix item alignment in icon view in the generic wxListCtrl.
 
+GTK:
+
+- Fix wxNotebook best size calculation.
+
 MSW:
 
 - Fixed regression with initial focus in the dialogs in 2.9.3.