Make blocking consistent

[cbrnr]

Fixes #13955.

Here's what I changed to make the blocking behavior consistent:

• plot_evoked_topomap: removed hardcoded block=False (so this is now consistent with other plotting functions).
• plot_sensors (plus corresponding methods): changed block=False to block=None (here the argument is useful to force blocking when needed for kind="select").
• Spectrum.plot_topo, plot_psd_topo methods, plot_raw_psd_topo: deprecated the block argument since I don't think these functions need it.

So the behavior now is as follows: Since show=True by default, this essentially means that the functions call plt.show() internally. This shows the figure and blocks by default. Blocking can be changed via %matplotlib in IPython or plt.ion() in the regular Python REPL. I'm not sure about Jupyter notebooks, but I think whatever they do shouldn't have changed (they probably do not block by default).

[larsoner]

Okay for me, @drammock did you want to look?

[wmvanvliet]

now that show has its own docdict entry, there are some visualization functions whose docstrings should use it. That can be a separate PR.

[wmvanvliet]

@drammock If you're happy with the docstring now feel free to merge.

[drammock]

Thanks @cbrnr !