Drag n drop behaviour of faces

This merge request implements a simple way to change the name of a face by improving the behaviour when drag and dropping a "face" under the "people" view. A possible solution was described in bug: 415880.

Now, you can simply drag and drop a face to a person album to change the name. As with other tags, you can also overwrite the thumbnail of an album by drag and dropping a single element of the album onto it.

Moreover, I disabled the options (like "group here") when drag and dropping a face onto another face in the "people" view, as this makes no sense.