Maestro: Expression editor should include a method to list column values

[ksgeograf]

It is very common to add filters like: 'Column' = "value"

The new expression editor now supports the "Column" part, but it should also be possible to read the "value" part.

This can take a long time as it is a query against the FeatureSource, so the user should actively retrieve the list of values.

[jng]

What if a certain provider doesn't support distinct value fetching (slim chance but possible). Brute Force?

[ksgeograf]

I would try to use the aggregate function to retrieve the (distinct) values, and if it fails, then fallback to fetching all values, and manual filter.

[ksgeograf]

Fixed in r4202, except the "distinct" part.