[Post-1.4] Tagxtag triggers

This is something I wanted to do since some time. It is not that easy to keep consistency on the database between tags, tagged_images and tagxtag. Using triggers makes this safe. It is even possible for developers to add a tag into tags table and get all things right. No need to mess with tagxtag.

I also foresee that this will be nice for the lua backend as adding tags or removing tags from lua will be safe an easy. Likewise for attaching/detaching tags.

Please review.