Now that the MySpell code is used by various other applications, it would be
nice to be able to install directories to a shared location outside of the "data
directories" of each of them.

Maybe be the best way to go about this would be to let the spellchecker to
search for dictionaries in directory currently used *and* somewhere like
/usr/share/myspell. (A more radical alternative would be to *only* look in
/usr/share or similar.) The MySpell-based spellchecker code of other apps (e.g.
Mozilla) would have to be updated in a similar manner, of course.

Another question is whether the dictionaries.lst file is really necessary. It
seems like all its information is in practice included in the dictionary files
and file names themselves; all the public dictionary files have a root name of
the format <language>_<country> - so perhaps we could simply locate the files
via a normal directory search instead. This would probably make a
multi-directory setup easier to implement. I think this is the way it's done in
Mozilla already.

See also https://bugzilla.mozilla.org/show_bug.cgi?id=216382