Reduce large VARCHAR columns of the wizard sample databases to fit Firebird

The maximum size of a VARCHAR column in Firebird is 32765 bytes [sic].
We use the UTF8 character set and that means 8190 characters as each
character might take four bytes. The maximum size of a row in Firebird
is 64 KB [sic].

Avoids errors when creating sample tables using the wizard using just
the default values. If the varchar column lengths are under 8190 but
the total row size is too large you do get a relatively helpful error
message "new record size of <N> bytes is too big", but if you try use
a too large size for a varchar column, just some "SQL error code =
-204", "Impmentation limit exceeded", etc.

Change-Id: I17c2bed4fc098f4e159822f6f81242aae274da7b
Reviewed-on: https://gerrit.libreoffice.org/47232Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tor Lillqvist <tml@collabora.com>