gitweb: Introduce %actions_info, gathering information about actions
Currently it only contains information about output format, and is not
used anywhere. It will be used to check whether current action
produces HTML output, and therefore is displaying HTML-based progress
info about (re)generating cache makes sense.
It can contain information about allowed extra options, whether to
display link to feed (Atom or RSS), etc. in easier and faster way than
listing all matching or all non-matching actions at appropriate place.
Currently not used; will be used in next commit, to check if action
produces HTML output and therefore we can use HTML-specific progress
indicator.
Signed-off-by: Jakub Narebski <jnareb@gmail.com>