Implement data streams dialog that allows importing a stream of data

in CSV format.
It's possible to read directly from a URL
or to run a process and read its output.

Change-Id: I3266c3bb175b73f1023440cca40b2cfab5fcc201
datastreams: implement moving the import range down until the limit is reached

Change-Id: Iaaed4399a980697c37683d838fcb1f99208233e8
datastreams: add toolbar buttons to stop / resume importing

Change-Id: I601a4a174ea191c799623d29ed6eb229f9f372da