support stimulus-related plots

[d1manson]

Since everyone is getting into optogenetics, it might be nice to have some support for stimulus-aligned rasters (or whatever it is people use).

I'm sure whether the stimulus settings are general enough as to be able to do this, or whether you would need to give the user a fairly detailed interface for customizing the interpretation of the stimulus data. I guess you could in principle execute a user-written function, but that's perhaps not necessarily much easier than building a complex dialog pane.