Support custom ordering

[rgaudin]

Note: this would be for a future release

I realized playing with the table ordering that nautilus has two display modes:

• random (default) where file entries are displayed in a random order every time.
• no-random where the files are always listed in the same order, the order in which those have been found in the collection.json.

Eventually, we'll want to allow users to control the order in which files are added to the collection. It would probably be through an ordering column on the DB and on the table in UI and a drag-to-reorder feature.

[FledgeXu]

I like this drag idea. Since we have the Edit and Upload of those two modes, we can allow users to drag to adjust the order in Edit mode.

[rgaudin]

Next nautilus version will drop the random feature so ordering (via drag&drop) will be more important.