Rename completed to stopped (as it is not only emitted
on completion). Ensure that the started and stopped signals
are emitted at the right times. Document the signals.

Task-number: QTBUG-14968
Change-Id: Icd3babcef2c9e544476592a26e6b9e58a21ebe95
Reviewed-by: Yunqiao Yin <charles.yin@nokia.com>