Search in the preview pane

Hi guys, it would be great to be able to **search** for text in the **preview pane**. In particular, selecting text in the preview pane and then hitting `Cmd+e` and `Cmd+g` is what I miss greatly. Regular search with `Cmd+f` too, of course.

Thanks for considering this request
Rudy

PS: Sometimes I use MacDown as a viewer with the editor hidden. Having no search hurts the most then.

No file or test is named. Start by tracing the macOS preview pane and its keyboard handling, then verify Cmd+f, Cmd+e, and Cmd+g when the editor is hidden. Done means text search and next/previous navigation work in the preview pane.

Written by the indexing model from the issue text.