RFE: status indications as to phase of a capture

[jwise]

On my DHO4000, it can take quite some time to capture, and the UI does not provide any indication that something is happening when I do. It would be nice for the UI to provide status as to what each instrument is doing (in multi-instrument settings), or what The Instrument is doing (if ngscopeclient is used to represent a single instrument). One reasonable place for this might be in the parameter status indication suggested by #760; in that case, the dho4000 may have a box saying 'stopped', or a box saying 'waiting for trigger', or a box saying 'downloading waveforms' (or each channel may have a box saying 'downloading waveform'!), which would indicate when I press the 'capture waveform' button that the system heard me and is doing something, it's just not done yet.

Note that this is not #79 (that is associated with the UI hanging; in this case, the UI is perfectly responsive, but it takes many seconds to download a waveform from my DHO4000).

[jwise]

@azonenberg notes that "RemoteBridgeOscilloscope based drivers generally always report 'triggered' and then block in acquireData"; it may be worth adding yet another state for 'busy, but we don't know exactly how'.

[jwise]

Also it would be nice for drivers that have a sense of what percentage they are in downloading to communicate that (i.e., rather than just saying 'downloading', provide to the UI information like 'downloading, channel 1, 33%').

[jwise]

I mocked this up here: https://codepen.io/jwise0/pen/eYqNdeM

[jwise]

Fixed by #765.