Don't set initial label in wxNativeWindow on OS X

Differentiate between setting the label from SetLabel() and from
SetPeer() (i.e. initially), because some native controls shouldn't have
an empty string set as their intitial value.

Override SetInitialSize() to be empty for wxNativeWindow in order to
prevent label-setting issues when an unknown, caller-controlled native
widget is attached. Reverts db9baf9aa5,
which was the previous attempt to fix this, but was causing too many
problems elsewhere.

include/wx/osx/core/private.h

@@ -288,6 +288,8 @@ public :
 #if wxUSE_MARKUP && wxOSX_USE_COCOA
     virtual void        SetLabelMarkup( const wxString& WXUNUSED(markup) ) { }
 #endif
+    virtual void        SetInitialLabel( const wxString& title, wxFontEncoding encoding )
+                            { SetLabel(title, encoding); }
 
     virtual void        SetCursor( const wxCursor & cursor ) = 0;
     virtual void        CaptureMouse() = 0;