ttools: use Math.log10
ttools: add time plot.

This is a new plot type, resembling the Plane plot but with the
horizontal axis representing a time coordinate.  There are some
other changes too.  This plot type may be upgraded in the future
to accommodate multiple stacked plots with a common horizontal axis.
topcat: add spectrogram layer control

This adds the required infrastructure to plot spectrograms,
but none of the available plot types currently has spectrogram
layers as an option.
topcat: add time plot type.