Plot topomap

[vferat]

Is your feature request related to a problem? Please describe.
When annotating bad segments and bad electrodes, it is often useful to have a look at the topography at a given timepoint (or the mean topography over several timepoints). It is not currently available in MNE, but we could take advantage of mne-qt-browser to implement it.

Describe the solution you'd like
Implement of new tool "plot topomap" which plots the topomap (mne.viz.plot_topomap) at the current timepoint (green line), for a selected time period , or the selected annotation in annotation mode. The general behavior would be similar to the interactive part of raw.plot_psd which allow to plot topomap of a selected frequency range.

The plot could be on an new window ( similar to the one used to plot channel locations) and update data automatically when the selected data changes.

Describe alternatives you've considered
Just a button to create a new figure which plots topomap at current location (no automatic updating).

Additional context
The feature is currently available in Cartool:

[larsoner]

Agreed we should have something like this. I think we should reuse the evoked.plot(...) code for this -- there is time-span-selection-to-topomap-plotting code that lives there already -- and it should work for mne-qt-browser and the matplotlib backends. I thought we discussed this already somewhere but I can't find it offhand...