#220: Improve browser layer support

At the moment we have support to manage views per theme using the plone.theme package. However it is just as important to be able to only enable views if a product has been installed in the site using the quickinstaller. This is possible using plone.browserlayer, but that is not shipped with Plone 3. A number of third party products already depend on this package and it is expected that that number will grow to the majority of Plone 3 products.

Either include plone.browserlayer as is, or merge it into plone.theme. In both cases more documentation explaining how to use the provided functionality will be needed.

plone.browserlayer already exists and is used in production.

Martin Aspeli, who wrote plone.browserlayer.