topcat: improve match plot expression construction

The match plotter has to guess to some extent about the expression
to use when it creates a new layer, since it does not understand
the expressions used in column selectors and the column names may
change between input and output tables.  I've improved the guessing
a bit.
topcat: just paint navigation decorations on top of plot

Simplify the painting of plot visual feedback decorations; just paint
them on top of whatever is currently visible rather than creating a
new PlotJob including them (just repaint, don't replot).
This makes them much more responsive and simplifies the logic
in various places.  It might possibly mean that you get an
inappropriate decoration for the plot you're viewing if plotting is slow,
but experiments show it doesn't tend to happen.
topcat: fix bug relating to BoxLayout

Peter reported a stacktrace "java.awt.AWTError: BoxLayout can't be shared"
at a Container.removeAll call, java6 but not java5.  I can't really see
what it was complaining about, but rewording the code seems to have
sorted it out.