Expose a toggle to enable import semantics when opening/logging RRD files

[teh-cmc]

RRD files do not have import semantics, they always use open semantics (i.e. they will always log their data to their original recording, rather than importing it in another one).

That's because RRD files embed their own recording metadata, which takes precedence over everything else.
You will see the same behavior when you try to drag-n-drop RRD files in the viewer.

We should offer a way to explicitly ask for importing semantics when opening/logging RRD files.