Enhance Labeling GUI with MNE-QT Backend

[adam2392]

Just to leave a comment for the next issue on v2.0 of the GUI:

The next version of the GUI would require a modular QT MNE backend.

1. This would require a PR as Martin suggests to BrowserBase to be able to sub out the actual data for the plotting. The API would probably be something like browser.replace_data(inst).
2. Then a PR to mne-qt-browser to add the capability inside the MNE QT browser
3. Finally, we would update the timeseries widget accordingly in mne-icalabel

Originally posted by @adam2392 in #66 (comment)

[mscheltienne]

@marsipu @adam2393 It looks like a lot of work for a niche use case of the browser. ica.plot_sources(self._inst, picks=[0]) will return a QtWidget. If I read correctly the _pg_figure.py, this widget browser should have a browser.mne.viewbox attribute and a browser.mne.plt attribute. And we should be able to swap the trace for new data through one of them.

But I have to get my head back into pyqtgraph.. I haven't used it for a while, and it's not straightforward to me in which argument the traces live in.