Remove const modifier from QBluetoothLocalDevice signals.

Those signals were added in 5.3 and therefore it is still possible to
change them. At the same time fix the documentation for the signals
and its related function

Change-Id: Ide0f7b07cf624f9692f830a2534b224212da223a
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>