Introduce the concept of a "device process".

Provide a QProcess-like abstraction that can be used
to implement processes running locally or on a remote
device. Objects of a concrete class implementing the functionality
are created by IDevice objects.
Current implementations are:
     - Local execution (QProcess-based), provided via the DesktopDevice.
     - Remote execution via SSH.
     - A specialized case of the former for remote Linux systems (provided by
       LinuxDevice).
The latter is already being used in a number of places. As a result, lots of
code dealing with details such as setting the remote environment could be
moved to a central location. These things are no longer the concern of whoever
is wishing to run a remote process.

Change-Id: I919260ee6e77a020ca47226a4a534e7b8398106f
Reviewed-by: hjk <hjk121@nokiamail.com>