Add an interface for getting "standard" file metadata.

Add an interface for getting "standard" file metadata.

Currently file metadata is handler dependant. However they usually
end up extracting the same type of data (author, date, etc) plus
one or two handler specific things. This adds a handler independent
interface for getting metadata that is likely to be common for all
types of file (At the moment, this is the exif/iptc/xmp information)

This commit used to also contain stuff adding parser functions,
which is now split to its own commit. This commit is needed
by a bunch of other commits, in particular I0d957891e0.

Change-Id: I43d9252f69dc5b8ba0b848cf40aa1b97329c85ae