ttools: import some more colour maps

These ones were pinched from Matplotlib.
I grabbed images from http://matplotlib.org/users/colormaps.html and
ran uk.ac.starlink.ttools.plot.LutSteal.
Kristin Riebe suggested some (more) from matplotlib would be good additions.

This commit just makes them available from the API, it doesn't
incorporate them into applications.
ttools: provide some more colour maps for aux/density shading

Add four more new options to the colour map list for shading.
These were pinched from matplotlib.
topcat: fix problem with activation action image viewer selector

If the list of potential image viewers (SAMP image.load.fits subscribers)
changed after the Image Viewer selection box in the Activation Window
had been used (clicked on so the contents were visible), subsequent
attempts to use that selection box failed, showing a blank list of
entries.  This was because the ComboBoxModel became invalid.
Reported by Jean Baptiste Marquette (10 Feb 2014).

Add code to forward ListDataEvents along from the SAMP subscribed client
list to the ComboBoxModel.