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;

src/osx/cocoa/nativewin.mm

@@ -30,6 +30,27 @@
 // implementation
 // ============================================================================
 
+namespace
+{
+
+class wxNativeWidgetCocoaImpl : public wxWidgetCocoaImpl
+{
+public:
+    wxNativeWidgetCocoaImpl(wxWindowMac* peer, WXWidget w) : wxWidgetCocoaImpl(peer, w)
+    {}
+
+    virtual void SetInitialLabel(const wxString& WXUNUSED(title), wxFontEncoding WXUNUSED(encoding)) wxOVERRIDE
+    {
+        // Don't set initial label, because the control was created by the
+        // caller and is already fully setup. And some controls (notably
+        // NSPathControl) assert if an unexpected string value, such as an empty
+        // string, is set.
+    }
+};
+
+} // anonymous namespace
+
+
 bool
 wxNativeWindow::Create(wxWindow* parent,
                        wxWindowID winid,
@@ -62,7 +83,7 @@ wxNativeWindow::Create(wxWindow* parent,
     // As wxWidgets will release the view when this object is destroyed, retain
     // it here to avoid destroying the view owned by the user code.
     [view retain];
-    SetPeer(new wxWidgetCocoaImpl(this, view));
+    SetPeer(new wxNativeWidgetCocoaImpl(this, view));
 
     // It doesn't seem necessary to use MacPostControlCreate() here as we never
     // change the native control geometry here.

src/osx/window_osx.cpp

@@ -353,8 +353,7 @@ void wxWindowMac::SetPeer(wxOSXWidgetImpl* peer)
         if ( !m_hasFont )
             DoSetWindowVariant( m_windowVariant );
         
-        if ( !m_label.empty() )
+        GetPeer()->SetInitialLabel( wxStripMenuCodes(m_label, wxStrip_Mnemonics), GetFont().GetEncoding() ) ;
-            GetPeer()->SetLabel( wxStripMenuCodes(m_label, wxStrip_Mnemonics), GetFont().GetEncoding() ) ;
         
         // for controls we want to use best size for wxDefaultSize params )
         if ( !GetPeer()->IsUserPane() )