Support background Bluetooth sensor readings

On mobile platforms (iOS, Android, WinRT) Qt pauses the main event loop which stops or queues new events. This is a problem for sensors which are often running in background or when the screen is turned off (heart rate, steps, speed sensor, blood presure, environmental sensors, etc.).

At the moment background sensor readings have to be implemented using native code for each platform which makes the Bluetooth module unneeded.

The Bluetooth module should offer means (callbacks, its own event loop, ...?) to continue measurements even if the main event loop is halted.