Simplify how backend methods are injected

[davinov]

Reviewing #504 raised the issue that the manner we tie the back-end service to the hosting store and the weaverbird module is quite complicated.
I think it could be simplified to remove this potential confusion between the two store.

IMO, this is also a required is we want to call more and more back-end operations from Weaverbird UI, therefore being able to handle large datasets.