Rich UI support for StreamBrowserDialog on oscilloscopes

[jwise]

This is a draft PR to experiment with some UI for #760 and #761, and in general, the concept of making the StreamBrowserDialog meaningfully richer in showing instrument and system status. Here is what I have right now:

Obviously there is going to be a lot of stuff to add and cleanup to do before this is ready to merge, and probably will include some scopehal changes, but this is the current UI metaphor set that I'm playing with. Title bars are still draggable, for instance, but other parts are not, and I think this makes some sense. Some things to consider at some point:

• Do something sensible if the Stream Browser is not wide enough to contain a badge
• Show channel properties
• Maybe make better use of wide horizontal Stream Browsers to provide more information in fewer lines (i.e., channel coupling?)
• Trigger settings seem like they are part of an oscilloscope?
• At that point, is it really even called a Stream Browser if copying Streams around is just a small part of its functionality?
• Should there be a UI affordance of 'things that you can drag' to indicate 'try dragging me to a filter graph'? (Think, for instance, Apple's Interface Builder has the little blue bullseye for 'draggable and droppable things'.)

[jwise]

Here's where @fredzo and I landed on this so far:

I think I'm going to declare this Ready for Review, though I admit that I haven't looked at the diff as a whole yet myself. But this is definitely something that other people should consider testing and playing with; there's support for Siglent, Rigol, and the Demo scope in the scopehal upstream.

[fredzo]

@jwise I think we should not show status badges on non-slow channels to prevent flickering effect: