Add a watch window

Created by: vaussard

Most modern debuggers offer a watch window, so you can keep track of a limited set of variables.

In Aseba, this could translate to a network-wide watch window, so you can even look at the variables of other nodes at the same time. Of course, the variables should be grouped by node (or maybe the user should be able to order them?), and an icon should inform the user about the status of this node / variable (running, stop, step-by-step,...) :-)