Initial ground work to let device items specify a different thumbnail location
Another step towards #29 (closed)
Cameras adhering to DCF can use the same URI to refer to different images over time, regardless of whether it is the same device or not. Manufacturers use the same URI naming scheme for all their products, so it is likely that two different devices made by the same manufacturer will have two distinct images at the same URI. Given enough time, once various counters have wrapped around, it will happen with images taken by the same camera too.
Therefore, it is desirable to not put all those thumbnails in the same bucket, and have some degree of separation.