Use an overlayed component for the topics picker

This prevents the topics file picker from consuming the whole screen
and makes it consistent with the standard file and buffer pickers.

helix-term/src/commands/typed.rs

@@ -1863,7 +1863,7 @@ fn help(cx: &mut compositor::Context, args: &[Cow<str>], event: PromptEvent) ->
                         },
                         |_editor, Topic(path)| Some((path.clone().into(), None)),
                     );
-                    compositor.push(Box::new(picker));
+                    compositor.push(Box::new(overlayed(picker)));
                 },
             ));