Customstyles: fix meta image inclusion (favicons, apple touch icons).

Includes these fixes:
- Use the ICustomStyles adapter - the annotation key was wrong and could never work.
- Override Plone's plone.links.favicon viewlet for not having duplicate <link>
  tags in the HTML anymore.
- Remove special handling in rules.xml which did not work anyway.

Also:
- Added tests.

@ninfaj Could you take a look?