Fix wxMenu::GetTitle() before the menu is appended to the menu bar.

The mnemonics conversion was not done correctly when the menu title contained
them but the menu hadn't been appended to the menu bar yet.

Closes #15461.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74731 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/menu.h

@@ -81,7 +81,8 @@ public:
     // Returns the title, with mnemonics translated to wx format
     wxString GetTitle() const;
 
-    // TODO: virtual void SetTitle(const wxString& title);
+    // Sets the title, with mnemonics translated to gtk format
+    virtual void SetTitle(const wxString& title);
 
     // implementation GTK only
     GtkWidget       *m_menu;  // GtkMenu