updated strftime lib. fixes #4449

Replace strftime with Intl ICU. Fixes #3573

This uses a class that maps strftime placeholders to the appropriate ICU
patterns. I am using the fallback-intl branch here which provides an
English-only

Replace strftime with Intl ICU. Fixes #3573

This uses a class that maps strftime placeholders to the appropriate ICU
patterns. I am using the fallback-intl branch here which provides an
English-only fallback when the intl extension is not available.

Core has only two places where strftime is used: dformat() and the
SimplePie feed parser. Both are adjusted with this patch. For the latter
a custom Item class had to be registered. For better separation all our
FeedParser classes have been moved to the Feed namespace where our
FeedCreator classes already reside.

Note that this will currently be a degration for users without intl as
it will fall back to date and not to the still available strftime.

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