No real changes but refactor wxSizerXmlHandler to make it easier to derive
from it by adding virtual IsSizerNode() and DoCreateSizer() methods. To add
support for a custom sizer class you only need to override them in
wxSizerXmlHandler subclass now.
Also document wxSizerXmlHandler which was not documented at all previously.
Closes#11845.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65889 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
