core: Revert "Remove MetadataExtractor class."
core: Revert "Remove MetadataExtractor class."

This reverts commit 4d530bd0347c69a95110c7d399372e19cdf83793.
libowl would drag in a hard dependency to GTK+ which is not desired

Conflicts:

    src/rygel/Makefile.am

GNOME/rygel,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,GNOME/rygel,GNOME/rygel,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,luisbg/rygel