Make a moving agent, dispatched from the backend

To prove that the backend actually connects to the front end, I want to implement some basic "agent", to move beyond the concept of just a cell.

This might also introduce the concept of "periods" or "time activations", but it probably won't need to be too robust.

The idea is that at time t=0, cell 0 is "activated", meaning it turns a certain color (anything but white).
At time t=1, cell 0 goes white, and cell 1 "activates" to a non-white color, and this continues until we all the cells have been activated and deactivated in sequence. The color and timing will be sent in messages by the backend, and the whole sequence will be kicked off by a single button click.