Work in progress: refactoring Reader to introduce a Panel class.
Refactored reader introducing a dedicated architecture for panels.

Careful: this will have regressions.
Fix several regressions.
Merge branch 'lens_next' into reader_refactor

Conflicts:
	src/panel_factory.js
	src/panel_specification.js
Wording.
Fix issues with scrolling and outline updates.
Fix router issue.

My lesson: an <a> element should not have an '#' href if you don't know
what you are doing.
Cleanup.
Fix TOC click handling.