XFit-style dipole fitting GUI

[wmvanvliet]

This adds a GUI to perform guided dipole modeling in the spirit of MEGIN's XFit program. This PR contains the base functionality needed to make the GUI useful. Useful enough to include in the next release of MNE-Python. The plan is to keep adding features in future PRs as well as some sorely needed speed improvements.

See here for a list of currently supported features: #11977
This PR depends on: #12071

Minimal example:

import mne

data_path = mne.datasets.sample.data_path()
evoked = mne.read_evokeds(
    f"{data_path}/MEG/sample/sample_audvis-ave.fif", condition="Left Auditory"
)
evoked.apply_baseline((-0.2, 0))
trans = mne.read_trans(f"{data_path}/MEG/sample/sample_audvis_raw-trans.fif")
cov = mne.read_cov(f"{data_path}/MEG/sample/sample_audvis-cov.fif")
bem = mne.read_bem_solution(f"{data_path}/subjects/sample/bem/sample-5120-5120-5120-bem-sol.fif")
subject = "sample"
subjects_dir = data_path / "subjects"

evoked.pick("grad")

# Quick and dirty. No MRI whatsoever.
g = mne.gui.DipoleFitUI(evoked)

# Slow and proper. With proper BEM model.
# g = mne.gui.DipoleFitUI(evoked, trans=trans, cov=cov, bem=bem, subject=subject, subjects_dir=subjects_dir)

Todo:

• Basic unit tests
• In-depth unit tests
• Proper documentation page
• Towncrier

[larsoner]

@wmvanvliet I pushed some tests and some minor fixes. Good to go from your end? If so I say let's merge and keep iterating in follow-up PRs! Updated example here.