ProjectExplorer: Introduce DeviceProcessSignalOperation.

Every device can now return a DeviceProcessSignalOperation,
which allows to kill or interrupt processes running on the
device.

Change-Id: Idaa04ebc767e09ca167fa033ed93860b9b81479e
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Reviewed-by: David Kaspar <dkaspar@blackberry.com>
Reviewed-by: hjk <hjk121@nokiamail.com>