Bytes should not be default
Fix for #719

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.
Merge branch 'FIX_FIELD_LIST_SLOWING'