RFE: cursors: allow multiple cursors

[jwise]

While I was debugging a power supply last night, I found often that I wanted to leave one set of cursors around as a reference for precisely what I was looking at a moment ago, and add another set of cursors around in a dataset (and maybe name each set of cursors). Even better, I would like to do computations on these cursors relative to each other (for instance, it would be nice if ngscopeclient could tell me not just period and frequency, but also if it could tell me duty cycle if I somehow tell it that two cursors are related); these are things I would write down in 'lab notes' if I had to, but my general philosophy is that it is better to show things in {code,UI,dedicated features} rather than tell about them textually in lab notes.