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

include/wx/osx/window.h

@@ -277,7 +277,7 @@ public:
     //
     // this is useful for a few Cocoa function which can work with either views
     // or windows indiscriminately, e.g. for setting NSViewAnimationTargetKey
-    virtual void *OSXGetViewOrWindow() const { return GetHandle(); }
+    virtual void *OSXGetViewOrWindow() const;
 #endif // Cocoa
 
     void *              MacGetCGContextRef() { return m_cgContextRef ; }