Move Objective-C interfaces into separate header files in preparation for being able
to use them from other source files.
No new or changed code. Work on behalf of Software 2000 Ltd.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@47388 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775