ENH/API/MAINT radically improved and pythonic sensor-topo API

To summarize, this PR

1) puts into practice some really overdue cleanup measures with regard to the `plot_topo_*` related code in viz, that is, improve its readability, replace hacks by pythonic / deignated matplotlib alternatives. It was somewhat painful but necessary, like a visit to the dentist.

2) Expose a fresh, new and ridiculously simple iterator API for creating custom plots on helmet / cap positions, documented in one new example.

3) With the proposed changes we won't need to add any other plot topo function in the future, e.g. `plot_topo_psd`, `plot_topo_decoding_accuracy`, `plot_topo_time_generalization`, `plot_topo_whatever`. (Had I realized this option earlier, I would not have added any of the topo functions we now have ...)

I hope you're going to like it.