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