Make it easier to define custom wxSizerXmlHandler subclasses.

Make it easier to define custom wxSizerXmlHandler subclasses.

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