Refactored TimeSeriesPanel preprocessing - the scaling/offsets

controlled by the usePixdim and plotMode properties are now performed
by the TimeSeriesPanel, instead of the TimeSeries class. This was done
because TimeSeries instances added to the list already had
scaling/offsetting applied, so it was potentially being doubly applied.
This is no longer the case, due to a new 'preproc' option in the
PlotPanel.drawDataSeries method.

Also added some melodic specific stuff to the OverlayInfoPanel.

Also fixed the overlay.guessDataSourceType to handle melodic analyses
that are located inside other melodic/feat analysis directories.

Also tweaked the new colourmaps.randomDarkColour function to be slightly
less dark.