Device Support: Move some functions from IDeviceFactory to IDevice.

Now that we have a polymorphic device class, we don't need to
attach all the virtual functions to the factory anymore.
In addition, the isUserEditable() function is gone, because it is
no longer needed in the presence of device-specific widgets.

Change-Id: Ie8f0be95653cb83e1618885b08cdd2aacf5dda59
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>