[API] Rename #dtype to #backend or something like this

[zverok]

Reason: in many cases (like NMatrix), as well as intuitively, container.dtype reads like "type of data stored in this container" (like numbers or string or what). But Daru's #dtype means "storage backend", which is confusing.

[v0dro]

Agreed. How about container, or data_store? We'll also need to make some non-trivial changes in notebooks.