[Discover] Filter editor doesn't allow adding top-level boolean conditions when all top-level conditions have sub-conditions

[smokris]

Steps to reproduce

• In Discover, click the Add filter button
• Click the OR button
  • There are now 2 top-level boolean-or conditions, good
• On each condition, click the AND button

The structured filter editor panel now looks like this:

At this point, I'd like to add another top-level boolean-or condition, but the only way to do that is to delete one of the existing sub-conditions. (It's simple enough to delete and re-create empty conditions, but imagine I have already created a complex set of conditions with many is one of options selected — deleting and recreating that would be tedious.)

Proposed solution

Add a button to create another top-level condition of the same type as the existing top-level conditions:

Clicking that button would add another top-level condition:

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)