Define a 'Coupler' class to manage Remote IOs

Currently, remote IOs are not properly managed in SkyCAD.  A coupler must be defined as a separate PLC.  This is not only unintuitive, but the PLC-coupler can not be associated to the main PLC.

We need to introduce the following concepts:

- Add a 'Coupler' class as a sub-class of the PLC class

- Modify the 'GetModules' PLC query so that it will list modules that belong to both the PLC and any coupler object associated to the PLC.

- Make sure modules are numbered based on all PLC modules (direct children and coupler chidren)

- Modify the SubLayout query so that only direct children are listed (not coupler's children).  The query should also avoid listing the coupler.

- Ticket 4481 will need to be fully implemented to allow for numbering to be properly processed.