[core/output] Move widget/module handling inside output

The core.output module now manages the list of modules and retrieves the
widgets inside draw() itself. That way, details of drawing/updating
widgets are not visible from the outside anymore.

doc/NOTES.md

@@ -16,3 +16,5 @@
 
 ## Improvements
 - pango output (improve - maybe autodetect? see #531)
+- only update specific, affected modules when clicking
+- allow handlers to specify whether to update or not (e.g. scroll)