On screens with restricted space, it's useful to be able to add controls

On screens with restricted space, it's useful to be able to add controls
to e.g. the wxChoice control of a wxChoicebook. GetControlSizer
allows an app to do that, and we also add a control margin which
may or may not be respected by individual book controls.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38698 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775