Keynav focus disappears after GTK4 port

In !141 (merged), @pwithnall wrote:

Keynav focus seems to disappear in two places:

1. If keyboard focus is on the burger menu, and you press tab, the focus goes to the carousel arrow (which is fine). Press tab again and the focus disappears.
2. If keyboard focus is on the ‘Read guidance’ hyperlink, pressing tab does not move the focus to the application usage restrictions widgets.