Temporary table handling improvements

Use-case:
Most of our users are not allowed to modify the database schema, but it's handy for them to create temporary tables to pre-process their data before e.g. inserting them.

Suggested improvements:
- The "+ Table" view should have an option "[x] Temporary". Currently, you have to execute CREATE TEMPORARY TABLE SQL.
- The sidebar currently shows `pg_temp_`, it would be nice if these tables were shown in a virtual "Temporary" schema or ideally even at the top level.

Start by tracing the "+ Table" view and the sidebar's schema/table handling. Review how temporary tables and the pg_temp_ schema are represented, then verify that the view offers a Temporary option and that temporary tables appear under a virtual Temporary schema or at the top level.

Written by the indexing model from the issue text.