Make public headers compatible with Objective-C++ with ARC.

OSXGetViewOrWindow() virtual methods were defined in headers and were casting
NSView*/NSWindow* to void*, which the compiler complains about with ARC
enabled. Rather than writing conditional code in the header, move the
implementations into .cpp files. They were virtual anyway, so this is no less
efficient, and doesn't leave any problematic code in public headers.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74855 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775