Fix setting the title for the wxDirDialog on OSX

OSX 10.11+ doesn't actually display the title, so update documentation
to reference SetMessage instead. For pre-10.11 override the SetTitle
method to set the title of the NSOpenPanel instead of the window.

Also change so the directory is not updated unless it is provided.

Closes #15143.

include/wx/osx/dirdlg.h

@@ -47,6 +47,10 @@ public:
 
     virtual int ShowModal() wxOVERRIDE;
 
+    // MacOS 10.11 has removed the titlebar from the dialog, so this is provided
+    // only for compatibility with older versions
+    virtual void SetTitle(const wxString& title) wxOVERRIDE;
+
     virtual wxString GetPath() const wxOVERRIDE;
     virtual void GetPaths(wxArrayString& paths) const wxOVERRIDE;
 
@@ -69,6 +73,7 @@ private:
     void Init();
 
     wxArrayString m_paths;
+    wxString m_title;
 
     wxDECLARE_DYNAMIC_CLASS(wxDirDialog);
 };

interface/wx/dirdlg.h

@@ -71,6 +71,11 @@ const char wxDirDialogNameStr[] = "wxDirCtrl";
     @endcode
     instead of just using @c wxDD_DIR_MUST_EXIST style alone.
 
+    @remarks MacOS 10.11+ does not display a title bar on the dialog. Use SetMessage()
+             to change the string displayed to the user at the top of the dialog after creation.
+             The SetTitle() method is provided for compatibility with pre-10.11 MacOS versions
+             that do still support displaying the title bar.
+
     @library{wxcore}
     @category{cmndlg}
 

src/osx/cocoa/dirdlg.mm

@@ -73,6 +73,12 @@ WX_NSOpenPanel wxDirDialog::OSXCreatePanel() const
     wxCFStringRef cf( m_message );
     [oPanel setMessage:cf.AsNSString()];
 
+    if ( !m_title.empty() )
+    {
+        wxCFStringRef cfTitle(m_title);
+        [oPanel setTitle:cfTitle.AsNSString()];
+    }
+
     if ( !HasFlag(wxDD_DIR_MUST_EXIST) )
         [oPanel setCanCreateDirectories:YES];
 
@@ -159,6 +165,12 @@ void wxDirDialog::ModalFinishedCallback(void* panel, int returnCode)
         SendWindowModalDialogEvent ( wxEVT_WINDOW_MODAL_DIALOG_CLOSED  );
 }
 
+void wxDirDialog::SetTitle(const wxString &title)
+{
+    m_title = title;
+    wxDialog::SetTitle(title);
+}
+
 wxString wxDirDialog::GetPath() const
 {
     return m_paths.Last();