Reorder event things a little.

  Reorder event things a little.
    Two callbacks for page change and page changing.
    use signal_connect_after() for page changed event,
      as everywhere else, this should also make it
      possible to select a different page in the
      event handler.
    GetSelection just uses native call.
    Don't use m_skipEvent flag, disconnect from events.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@43051 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775