Use wxBaseArray instead of object array for wxAuiToolBarItemArray

This class is part of the public API, as it is used by the public
wxAuiToolBar functions, so we can't make it just a vector, but we can
still make it a wxBaseArray instead of using the old macro-based object
array for it.

Also document it.

include/wx/aui/auibar.h

@@ -254,9 +254,7 @@ private:
     int m_alignment;             // sizer alignment flag, defaults to wxCENTER, may be wxEXPAND or any other
 };
 
-#ifndef SWIG
-WX_DECLARE_USER_EXPORTED_OBJARRAY(wxAuiToolBarItem, wxAuiToolBarItemArray, WXDLLIMPEXP_AUI);
-#endif
+using wxAuiToolBarItemArray = wxBaseArray<wxAuiToolBarItem>;