picker snuffling

[marzeck]

This little snuffling helps to set parameters of the PhaseMarker from drop-down menus (phase, kind, polarity). It now allows to set the polarity of the phase. But to do so, the PhaseMarker module in pyrocko needs to be updated by another function called set_polarity. Will add it to another pull request.

[marzeck]

Hey @HerrMuellerluedenscheid,
I've updated the the picker snuffling for the polarity as it is implemented now in pyrocko. So the functions set_polarity and the occurrence of the polarity labels are consistent with the current pyrocko master branch, as described in #234.

Maybe it would be helpful to explain somewhere, that the "undecidable" polarity (labeled with "0") can be accessed by Shift + arrow up/down.