Color theme configuration/saving.

The default color theme is still 'spectral'.
Via Edit --> Preferences, one can select a color scheme and save the
settings to a configuration file (e.g. ~/.visram on linux).
The chart automatically updates based on the color scheme selected via
wxPython's pubsub.

create_graph in chart.py now accepts a color theme as an argument.

The color schemes are taken from matplotlib.cm.cmap_d.

ProcessWedges now store their depth in order to properly calculate their
color when arbitrarily changing color schemes.

The color map in chart.py is no longer global.
Updated screenshot