improve ica.plot_overlay for raw instances

Describe the new feature or enhancement

Currently when providing inst=raw, ica.plot_overlay transform it to an evoked for display. This doesn't really make sense when working with spontaneous data.
I would be great to be able to plot the overlay on the classic raw.plot() window. Each data channel would be represented by two superimposed traces of different colors: one representing the raw instance and the other the raw instance without the ICA components.
It would be useful as in this example, where we users are forced to manually apply ICA to a copy of the raw instance and then plot the 2 raw instances.

Describe your proposed implementation

Directly change the behavior of ica.plot_overlay(insta=raw) or create something like joint_plot(raw1, raw2) which allows to plot several raw instances with same info on the same plot.

Describe possible alternatives

A way to synchronize two raw.plot() windows, which allows to have always the same time window of the recordings in view.

Additional comments

This feature could also be implemented in mne-qt-browser

Start by reading ICA.plot_overlay and raw.plot(), then compare the behavior shown in the linked artifact-correction ICA example. Consider whether the change belongs in these entry points or in mne-qt-browser. Done means raw instances can be displayed together with synchronized views and distinguishable traces for original and ICA-cleaned data.