Use unique ID for local data sources

[forman]

Expected behavior

Every newly created local data source should be assigned a unique ID and a human-readable name. The ID should be part of the filenames used to persist local data sources, e.g. <name>-<id>

Actual behavior

Cate uses the data source name passed by users. If it is empty, a name will be generated. As most users don't provide a unique name or not a name at all, they'll end up with an error, saying the data source already exists, even if created with different subset criteria. This is both surprising and frustrating.

See also https://github.com/CCI-Tools/cate-desktop/issues/30

Steps to reproduce the problem

In the GUI:

1. Download data source
2. When done, download same selection once more, with other constraints, don't change name
3. --> Error

Specifications

Cate 0.9.0dev2

[forman]

in cate.util.im.image.AbstractTiledImage we use str(uuid.uuid4()).