Add OC_Migration_Content class to help app devs. Restructure OC_Migrate.
Add OC_Migration_Content class to help app devs. Restructure OC_Migrate.

shtrom/bookmarks,owncloud/bookmarks,alberanid/bookmarks,behrisch/bookmarks,owncloud/bookmarks,alberanid/bookmarks,tflidd/bookmarks,tflidd/bookmarks,behrisch/bookmarks,shtrom/bookmarks