This removes the cachekey parameter again and instead follows
@michitux's suggestion to trigger an event for the cache usage

Templates authors should support these to make the transition between templates easier. Templates which already have a sidebar could first check for $conf['sidebar'] and use it if it's defined, otherwise use their old own sidebar functionality to stay backwards-compatible.

removed 'view original' button from new media manager again (was added in b8a84c03) and made a link around the image instead, as that is a more minor change (as it should be during the RC phase) and is what was originally planned

Template authors need to update their _mediamanager.css

Thanks to Anael in the DokuWiki IRC channel for spotting the errors.