Provide API to distinguish between user and system locale

Code was incorrectly refactored in 937c7413. We need to know both the
user's locale as well as the language that the Windows UI is in. The
distinction obviously wasn't clear enough as it fooled me. Hopefully
this is a bit better. All users are updated in this commit.
Remove Open-Tran.eu from pre-installed web lookups

This is still in the documentation if someone wants to add something
similar.
Replace default config with a setup on localhost

The public demo server seems to be gone.
Add support for specifying the Postgres port
Remove support for old psycopg package
Keep a reference to psycopg2 in the object

This way we don't need to import globally and have the whole plugin
fail, but also, all methods have access to it. This fixes the incorrect
reference to psycopg in .query().
Move example into try block
Provide simple example that works without TinyTM

This works even if TinyTM isn't really installed in the database. It
also forces a delay to test what happens with possibly slow queries.
Better error handling

We don't want to keep querying a non-working server.
Convert TinyTM plugin to work asynchronously

This was blocking the GUI, and in the case of incorrect configuration
(like the default configuration) would render Virtaal useless while it
waits for the network timeout, even if someone just enables the plugin
by accident. This should now work well, although it wasn't tested with
a real installation.