Allow lasso selection sensors in a plot_evoked_topo

[wmvanvliet]

This allows you to select sensors with a lasso tool in the same manner as plot_sensors("select"). Upon selecting sensors, an SelectChannels UI-Event is published. This is a part of the planner XFit-like GUI, but has other uses as well. For example, we could by default make plot_evoked_topo open up a figure with the average of the selected sensors when you select them, similar to what happens when you click on a sensor.

At the moment, this doesn't work yet when unified=False. If there is a pressing need for it, I can add it, but perhaps this can also be left to a later PR.