topcat: replace icon in JNLP file

I'd missed use of the old topcat in JNLP.  Use the new one.
ttools: make the ANTIALIAS style key generic.

Move ANTIALIAS from FunctionPlotter to StyleKeys, so it can be used
by other plotters that need the same functionality.
ttools: fix polyline-drawing bugs
ttools: refactor line drawing in FunctionPlotter.

Use the LineTracer class instead of doing it with custom code.
ttools: rationalise plot2 line style selection.

Add a new class LineStyle used by both function and line plotting.
This allows to add antialiasing to line plotting and offers more reusability.
topcat: modify SwingHttpAuthenticator behaviour.

Now it re-uses the credentials from previous invocations.
This makes sense if you have to authenticate against the same
service several times in a row.  That shouldn't really happen,
but can do in certain multithreaded cases.  Could probably do this
better, but deal with it if there are complaints.