topcat: modify antialiasing again

Grid line and text antialiasing are now controlled separately;
grid in the Grid tab (where relevant) and text in the Font tab
(new option Antialias alongside existing ones Plain and LaTeX).
Controlling text and grid line antialiasing separately is probably
the right thing to do; it also means that antialiasing of Aux axis
and Legend labels get controlled by the font selection, which wasn't
happening (and would have been hard to do) under the previous
arrangement.