ENH: Intentional boundaries of MRI and CT

[larsoner]

Currently in the CT GUI code, the boundaries of the MRI/CT are visible only coincidentally (and for some data) because they show up only due to differences in 1) background / "zero" value, and/or 2) constant-value extrapolation when plotting. A more principled way to show the same information more clearly is:

1. allow setting the vmin/vmax of each image so that background noise can be effectively zeroed out / peak values of interest more visible
2. determine the bounding box of the given slice in each shown slice, and plot it around the slice

This will take a little bit of UI tweaking (vmin/vmax) and geometrical wrangling (putting in a matplotlib rectangle with the correct rotation, shape, origin, etc.) but I think it will be a nice aesthetic and usability boost.

Might be good to do after mne-tools/mne-gui-addons#14