Allow to add extra accelerators to wxMenuItem

These accelerators are not shown in wxMenuItem label, but still will
work.

Implement support for them in all major ports and XRC.

Co-Authored-By: Vadim Zeitlin <vadim@wxwidgets.org>

include/wx/menuitem.h

@@ -23,6 +23,8 @@
 
 #include "wx/windowid.h"
 
+#include "wx/vector.h"
+
 // ----------------------------------------------------------------------------
 // forward declarations
 // ----------------------------------------------------------------------------
@@ -122,6 +124,14 @@ public:
     // set the accel for this item - this may also be done indirectly with
     // SetText()
     virtual void SetAccel(wxAcceleratorEntry *accel);
+
+    // add the accel to extra accels list
+    virtual void AddExtraAccel(const wxAcceleratorEntry& accel);
+
+    // return vector of extra accels. Implementation only.
+    const wxVector<wxAcceleratorEntry>& GetExtraAccels() const { return m_extraAccels; }
+
+    virtual void ClearExtraAccels() { m_extraAccels.clear(); }
 #endif // wxUSE_ACCEL
 
 #if WXWIN_COMPATIBILITY_2_8
@@ -163,6 +173,10 @@ protected:
     bool          m_isChecked;      // is checked?
     bool          m_isEnabled;      // is enabled?
 
+#if wxUSE_ACCEL
+    wxVector<wxAcceleratorEntry> m_extraAccels; // extra accels will work, but won't be shown in wxMenuItem title
+#endif // wxUSE_ACCEL
+
     // this ctor is for the derived classes only, we're never created directly
     wxMenuItemBase(wxMenu *parentMenu = NULL,
                    int itemid = wxID_SEPARATOR,