Add support for dynamic (remote) sensors

Provide better support for dynamically-registered sensors.

While you can call QSensorManager::registerBackend at any time applications cannot receive a notification that the registered sensors have changed. There is also no unregister function so a long-running app on a system that dynamically registers backends for remote sensors may end up with a large number of registered backends that can no longer be constructed (ie. the QSensorBackendFactory will return 0 because the remote device is no longer available);

• Add an unregister function to solve part of the problem.
• Make an object to emit a signal for apps to be notified about changes to the available sensors.