Create wxStaticBoxSizer elements as children of wxStaticBox in XRC.

Create wxStaticBoxSizer elements as children of wxStaticBox in XRC.

As we now recommend creating the controls shown inside wxStaticBox as its
children, and not siblings, follow our own recommendation by doing just this
in XRC handler of wxStaticBoxSizer.

Also add a test of using a tooltip with a wxStaticBoxSizer element to the XRC
demo showing that tooltips do work now for them, unlike before.

Closes #14431.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71886 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775