Generalize coordinate formatting

@anizami - I have generalized a bit the formatting code. Basically the issue was that one could in principle have multiple overlay coords, so I thought it would be easier to simply allow the user to switch between either pixel, world, or overlay coords. I also changed the keys because o and p are already taken as matplotlib shortcuts (for example 'p' switches to pan/zoom mode). So I changed it to a single key, w, that can be used to switch between the different overlays. Does this work for you?

cc @Cadair