already has Gallery 3 tables. Otherwise, permit it to continue.
We key this off of the existence of the g3_items table. Theoretically
it's possible that the g3_items table is gone but other tables are
still there, which could mess things up. I'm not going to worry about
that for now.
Fixes ticket #175
creating tables with a table prefix.
There are still some queries that haven't been converted, so don't
start using prefixes yet. However, if you do, you can login and
modify the user profile.