Conversion of devicegraph to json

[ancorgs]

Pull request created to discuss some aspects about the code organization.

1. I created the concept of Section for stuff that used to be D-Bus interfaces (Drive, Block, Filesystem, etc.) But I didn't group all the sections at a separate namespace because I felt that Agama::Storage::DevicegraphConversions::ToJSONConversions::Section::Block was already too much.
2. For symmetry with the other conversions, I created a DevicegraphConversions::ToJSON class and a DevicegraphConversions::ToJSONConversions::Devicegraph one. But I find the second to be unnecessary. It's just one extra level but with the same responsibilities and API.
3. I decided to nest the partitions directly on the "partitionable" device instead of doing it inside of :partition_table (that is, inside the PartitionTable interface). That's a change compared to the D-BUS interface (they are nested within PartitionTable there). It kind of makes sense from the organization point of view and also from the code point of view (see below), but I'm not 100% sure.
4. I separated the concept of sections and nested devices. There is indeed a semantical difference. But honestly I only separated the concept to avoid some kind of "circular dependency" (it is not elegant if the sections create devices because then ToJSONConversions::Device would require the sections and some section may require ToJSONConversions::Device).

[coveralls]

coverage: 63.767% (+0.2%) from 63.577%
when pulling 40e13a3 on ancorgs:json_devicegraph2
into a74ccf2 on agama-project:master.