qtFred Feature Request: Scene Browser Filter by Class

From Shiv on Desmos: Also, I think it'd be super cool to have an ability to filter scene browser elements by a ship class name

I put effort for this request to medium, because I'm not exactly sure how much effort it will take, but I think it would take at least some new UI on the scene browser on top of adding the filtering to the browser.

Start by locating qtFred’s scene browser implementation and reviewing how its elements are presented. Define the intended filter UI and how filtering by ship class should behave, then verify that matching elements are shown without disrupting the existing scene browser.

Written by the indexing model from the issue text.