Add ability to change database root folder

[jveitchmichaelis]

If the database file is moved, the internal (relative) image paths will be incorrect. We should have the ability to update the location of the database relative to the images.

Possibly a method to do this would be to ask the user to locate a single asset (e.g. an image) and use that to infer what changes should be made to the rest of the paths in the database.