Outline: Add command to quickly search headings in active file

Use case or problem

The new search field in Outline view added in v1.5.2 is excellent and a really nice improvement.

It would be great if this were accessible via a command bound to a hotkey.

Proposed solution

Add a command which can be bound to a hotkey for quick access.

Current workaround (optional)

Clicking or in-file search.