add match preview tab 2

You mean [0]?

Only one specimen per generic alias is to be displayed. So we need
choose correct family and select unicode script somehow.

> > +          installed_families_from(@fcstate.fpl[generic_alias])[0]

First family matched by fontconfig for given generic alias will be
returned by fontconfig. This can change while amending setting in
second tab, tough.

> > +        @current_families[generic_alias] ||= match_family(generic_alias)

If no from the families in FPLs in second tab is installed (or no
family there at all), then distribution FPL is used (=> no change 
for displayed font so far).

> > +        scripts = font_scripts(@current_families[generic_alias]);
> > +        @current_scripts[generic_alias] = scripts.keys[0]

For given family, unicode scripts it contains are sorted according to
its coverage. So first is chosen on the begining, user can choose
other script later.