Additional support for Modbus data outside the DPM

The stack supports a new feature which allows registering additional so called 'windows' in server IO mode.

'Windows' are separate Modbus Register areas which can be handled in the host application by packet communication via mailboxes.

This  feature allows creating devices where fast Modbus data can be exchanged via dual port memory and additional Modbus data e.g. parameter data can be exchanged via mail boxes directly with the host application.