Allow hiding the table.colname fields from ad-hoc filters

[punkeel]

This is a follow up to the discussion in #75.
Today, the adhoc dropdown will list all the columns. twice: once with the table name prefixed, once without.

I often create dashboards reading from a single table (or multiple tables, but all using the same schema) and would benefit from hiding the [table]. prefix.

This doesn't seem doable using adhoc_query_filter.