MNT: window and layout in GUI API

[GuillaumeFavelier]

This PR refactors _QtWindow:

• adds support for custom central layout
• adds _window_create to spawn a window 'without pyvista support' (instance of _MNEMainWindow)
• adds the option theme_support to opt-out from automatic theming

and _QtLayout:

• adds support for grid layout

Here is a code snippet to try:

from mne.viz.backends.renderer import _get_renderer

renderer = _get_renderer()
renderer.show()

window = renderer._window_create()
window.show()

Further consideration:

• With more refactoring it should be possible to use _get_renderer() to spawn _MNEMainWindow instead of using _PyVistaRenderer. This more important matter is about about having a GUI API which does not depends on pyvista.

It's an item of #10565

[larsoner]

Sounds reasonable, let me know when to look!

[GuillaumeFavelier]

This is delayed until #10565 (comment) is resolved.

[larsoner]

Okay, let me know what PR to review whenever one of them is ready

[GuillaumeFavelier]

Assuming the CIs return green, this is ready for review @larsoner

[GuillaumeFavelier]

The failure on Test Windows 3.9 pip does not seem related:

================================== FAILURES ===================================
________________ test_spatio_temporal_cluster_adjacency[Numba] ________________
mne\stats\tests\test_cluster_level.py:576: in test_spatio_temporal_cluster_adjacency
    spatio_temporal_cluster_test([data1_2d, data2_2d],
<decorator-gen-336>:12: in spatio_temporal_cluster_test
    ???
mne\stats\cluster_level.py:1357: in spatio_temporal_cluster_test
    return permutation_cluster_test(
<decorator-gen-333>:12: in permutation_cluster_test
    ???
mne\stats\cluster_level.py:1123: in permutation_cluster_test
    return _permutation_cluster_test(
mne\stats\cluster_level.py:983: in _permutation_cluster_test
    H0 = parallel(
mne\utils\progressbar.py:157: in __exit__
    os.remove(self._mmap_fname)
E   PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'C:\\Users\\VSSADM~1\\AppData\\Local\\Temp\\tmp_mne_progky0ehjn_'
--------------- generated xml file: D:\a\1\s\junit-results.xml ----------------

[larsoner]

That looks like a replication of #10648 on CIs, the first time I've seen it! Agreed it's unrelated, though

[larsoner]

Thanks @GuillaumeFavelier !