set_parent
Description:
Sets the parent of this to parent.
If this already has a parent, then this call will fail. The parent will be set automatically when using the usual methods to add a child menuitem, so this function should not normally be called directly
Parameters:
| this | The Menuitem for which to set the parent | 
| parent | The new parent Menuitem | 
Returns:
| Whether the parent was set successfully |