Date keys rework: plugin changes

[Guillaume Emont <guijemont igalia com>]

As promised in [1], here are the updates needed for the plugins to work with
the date key changes in core.

I think most of it is straightforward; the only particular case is that for
bookmarks, I stopped exposing the date/time that was exposed (which was the
time at which the media was bookmarked), since this does not fit in the set of
keys we have, and I feel it isn't really needed anyway.

Cheers,

Guij

[1] https://mail.gnome.org/archives/grilo-list/2011-June/msg00043.html