Add filter_sql parameter to sql tables.

When working with SQL tables, there is currently no capability to filter rows. Frequently I need to filter rows by an entity_table in order to only link to the relevant rows matching a given entity_id.

This patch adds a filter_sql parameter to SQL Table sources. The table is then accessed as a subquery with the filter_sql applied in a WHERE clause (which falls back to 1 if not provided).

Working well and seeming not to break anything else on my site - further testing obviously welcomed!