ttools: fix up SUN/256 version history

I had been adding history changes to the list from the last release,
rather than adding them to a new list.  Fixed.
topcat: refactor WrapperDataStoreFactory

Move the generic wrapper class from topcat to ttools plot2 classes,
and change the interface to be a bit more general: pass the table
not just the row sequence.
topcat: cancel unwanted plots more aggressively

A plot job being executed is now terminated if another job is started
and that has different characteristics (e.g. different layers or data)
other than just being a modified pan/zoom or similar.
ttools: tidy up job submission code in PlotPanel
topcat: more responsive large/slow plots

When navigating (pan/zoom) round plots that take more than a significant
fraction of a second to draw, a plot of an evenly spaced subsample is
prepared during the navigation, and displayed at intermediate positions.
This can give a somewhat jumpy appearance to the plot during navigation
(the number of points changes according to whether you're within a
navigation sequence or at rest), but it gives you better visual feedback
overall, since you see frames more often.
ttools: fix axis labelling bug

Linear axis values were getting labelled as (e.g.) 5030 instead of 0.5030.
Fixed.
topcat: add Sketch option in GUI to control intermediate frame option

The Plot menu now contains a toggle button to control whether
intermediate frame drawing is on or off.