Several usability issues and/or enhancements

[cbrnr]

Today I played around with the browser a bit, so here are a couple of issues that I've discovered and some suggestions:

Issues

1. Icons are non-retina with PySide2 and PyQt5 (they look fine with PySide6 and PyQt6), solved by Use HiDPI icons with PyQt5/PySide2 mne-python#10956
2. Showing/hiding the channel overview is extremely slow; I even managed to completely destroy this action once (meaning that clicking on it did nothing, but everything else worked normally).
3. Scrollbars are not updated immediately after showing more/fewer channels (have to press a second time).
4. Is it not possible to show a specific number of channels? In my example, it's either all 3 or 1 channel, but 2 is not possible. Bug or feature?
5. If the toolbar is hidden (right-click, deselect Tools), there is no way to restore it.

Enhancements

1. It would be nice if the channel overview was resizable (e.g. by dragging the border between the signals and the overview), because it takes up a lot of vertical space.
2. The status bar is currently useless. It should either be hidden or populated with some useful information (such as the time under the mouse cursor etc.). I think even if it gets some useful information, there should be an option to show/hide it.

I'm happy to work on some of these things, but I'd like to hear your opinions first.

Example to show some of the problems:

import mne
from numpy.random import default_rng

mne.set_config("MNE_BROWSER_BACKEND", "qt")
fs, nchans = 250, 3
rng = default_rng(42)
data = rng.standard_normal(size=(nchans, 50 * fs)) * 5e-6
data[0] += 10
info = mne.create_info(nchans, fs, ["eeg", "misc", "stim"])
raw = mne.io.RawArray(data, info)
raw.plot()

[larsoner]

Those all seem reasonable to me

[hoechenberger]

See also those two stalled PRs where I intended to improve UX, but never had enough time to finalize:

#126
#127

[marsipu]

@cbrnr Thank you for pointing these out
I added the ones about the Overview-Bar to #27.

The status bar is currently useless. It should either be hidden or populated with some useful information (such as the time under the mouse cursor etc.). I think even if it gets some useful information, there should be an option to show/hide it.

It displays time and data information when the "crosshair"-mode is toggled with x. That could be a part of #125