Fix for #719

This fix is there to make the table view display faster when there are a lot of fields, without too much overhead in the general case.

In our logs, we have (because they are not yet cleaned up!) more than 100 fields, and a deeply nested and large structure. Flattening the json to extracts the keys for the fields was taking quite a lot of time in the browser.

This patch delays the fields loading to when we actually need them.


Several changes in the way the Tables enumerate its fields.

- No longer flatten the source to display the table view.
- Use fromSources filter to dig into source hash structure to find
fields when displaying the view

- Trigger actual field loading on fields list opening
- Use current search result to update the field list when the list is already visible in view at search time

The result is that provided the fields list view is closed, the widget behaves much quickly.