- Remove redirection of setDisplayName() via DeviceManager. This
inflated the API (method + signal) only for the settings widget, which
can handle this case perfectly well by itself.
- Make the public API Id-centric. Indexes were used in a lot of places
for historical reasons (the device manager was a model once), but they
made the API inconsistent and sometimes even plain stupid (notably the
deviceRemoved() signal).

Change-Id: I77601ac05726d1cd841ae213397f26048dddc585
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>