[Export CSV] Allow column names to be customized on export

[gino-m]

On "Export observations":

1. Scan all features being exported for keys in properties.
2. Show dialog with table of all property names and form questions on the left, with generated column names on the right (e.g., "Total cost (USD)" -> "total_cost_usd").
3. Duplicate column names should not be allowed, indicated in red/alert style.
4. Allow user to edit column names.
5. Add checkbox for "Set at default", selected by default on the first run. Save mapping to layer when ticked.
6. Pass mapping to export function, use in results.

@kdyson Wdyt? This would be a longer-term solution.

[kdyson]

That sounds reasonable.