Move files with external modifications

[Pablo Casas Estévez]

External modified files are stored at BackingPhotoTable Table.
Id at BackingPhotoTable is the same ID at PhotoTable's editable_id field.
If a file has not editable candidate, editable_id value is -1

While moving files, editables files should be also moved within the original file and informed at DB (BackingPhotoTable).

[Pablo Casas Estévez]

Solved on commit ff0e8ef. Now editable photos are also moved alongside its originals.

Filenames are renamed according Shotwell style _modified or _modified(n) if file exists.
Folders are collected in order to be cleaned, so they are not left empty after process.
Option insert title metadata into editables photos has not been implemented.