use correct FeedOptions methods. fixes #4203

This makes feed.php use the correct methods for setting the feed type
and content-type header. It also adds the missing type definition for
RSS 1.0

Reintroduce the FEED_DATA_PROCESS event

As explained here:
https://github.com/dokuwiki/dokuwiki/issues/4191

�� Rector and PHPCS fixes

feed: use default favicon as very last resort

The dokuwiki logos we deliver in the wiki namespace are preferable.

feed: set item date. fixes #2960

The date was completely missing in the recent refactor. This readds it
and uses a unix timestamp as suggested in #2960

style fixes

feed: set tagline as subtitle. fixes #2239

Feed creation refactoring

This breaks up the humongous functions from feed.php into multiple
classes. To keep compatibility with existing Plugin events, the basic
principle of how the feed is assemb

Feed creation refactoring

This breaks up the humongous functions from feed.php into multiple
classes. To keep compatibility with existing Plugin events, the basic
principle of how the feed is assembled has not been changed:

* depending on the given mode and other options lose arrays of items are
gathered
* these items are then converted (again based on the various options)
into proper FeedItems
* the FeedItems are then added to the Feed

The conversion from loosely typed item data to something more structured
is now done by the FeedItemProcessor classes. Some very basic tests have
been added. It does not cover erverything but covers more than before
(which was nothing).

Manual testing before merging this is highly recommended. I am not
confident that I ported over everything correctly.

No new features have been added, but especially media support could and
should be improved in the future.

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