WIP: Add raw plotting

Currently it plots raw data that has been preloaded (or not), with scrollbar-like functionality across both channels and time, as well as up/down/left/right arrow key functionality. 

It has the guts needed to selectively apply projectors, but that will be added in a subsequent commit (button in the lower right for adding another plot / dialog for selection).

I think it also has some adjustments to be made in terms of layout tightness, but this should be good for a first pass.

People are welcome to try it -- should be able to do `mne.viz.plot_raw(raw)` and use it.