Looks ok, but the strings on checkboxes are a bit long and they don't wrap by default. I'm now checking if there is an easy way to make them wrap, or if the checkboxes should be replaced with something else, like label + switch.

I was hoping to have them as descriptive as possible, and even shortened them because they were too wide for my prefs window. The switch + label would hurt accessibility, unless there is a way to associate them. But it appears you can just do checkbox = Gtk.Checkbox() and then checkbox.add(label), where label is a wrappable label. I will update the patch later.

I have piece of code which sets word-wrapping on for all checkboxes in my soon-ready PR for narrow preferences dialog, so no need to do it manually here.

Maybe add a default youtube-dl format string here, as discussed in #1263?

The CheckButton line wrapping code is now submitted in #1274.

We can implement the #1263 fix in a separate PR that adds the setting, functionality and UI.