Heart Listener example added

Bluetooth Low Energy example for connecting and receiving measurements from the heart belt.

Documentation added too.

[ChangeLog][QtBluetooth] Heart Listener Bluetooth Low Energy heart belt example has been added

Change-Id: I1b690827af85411c962eb09aa87936f1fb7d9246
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Fix for enabling BLE notifications on Linux implemented

For some devices it was not possible to enable notification because of the if clause.

One warning in the constructor fixed.

Change-Id: Ic11ce4109829eeb2022490f256f4e873ef3073a5
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>