Allow grouping of signals

I wanted to give it a try first, but since I started to use surfer a bit more I noticed the lack of grouping signals. It nice when looking at a bigger system to be able to quickly hide a few signals, depending on what exactly one is looking at is really helpful. For me it's less a thing when jumping around a bit, but when I'm consistently looking at the same (bigger) component it's nice to have a setup with a bunch of "important" signals that I'm used to. (E.g. I was looking for issues in a signal processing component lately, where I often have 50-60 signals in some groups that I'm used to at some point w.r.t. relative position/colors/etc. - it often comes back to similar signals...)

To be clear, that'd be equivalent to the grouping feature in gtkWave (though it doesn't have to be as big ;-))