Improve the RWS subscription implementation

[jontje]

The current implementation for using RWS subscriptions is a bit unwieldy. This can be made more user friendly.

As it is right now, when a subscription event occurs, then the user is required to handle a XML document and process it themselves. Also, if the event content is inconsequential, then the XML document handling is just in the way.

Simple example (for the latter case)
Robot controller (RC): Requests an external computer to do some work when needed
External computer (EC): Performs some computational heavy calculations (e.g. image processing)

1. The EC can register a RWS subscription (e.g. on a IO-signal), and wait for a subscription event.
2. At some later time the RC sends a notification (e.g. triggered by the RAPID program), and continues with some other task until the results from the EC are really needed.
3. The EC receives a subscription event and performs its task, and then it can send the results back to the RC (e.g. via the methods for writing to RAPID variables). The EC can then start waiting for a new subscription event.
4. Finally, the RC processes the results and continue with its program.