Implement QBluetoothLocalDevice::allDevices() method

Implemented enumeration of local Bluetooth devices on Windows.

Tested on Windows 8.1 32 bit with several USB Bluetooth dongles,
based on "BCM920702" chip (from the Broadcom Corp) and on "CSR8510 A10"
chip (from the Cambridge Silicon Radio Ltd), using Qt5 (MinGW).

Important note: For some unknown reason Windows refuses to correctly work
with more than one USB Bluetooth dongle at the same time.
When the second dongle is inserted the device manager displays
"This device can't start.(Code 10)". This issue can even be reproduced
when the two USB devices have an identical chip, or two devices on
different chips are plugged in. Others combinations of devices and
their quantities (more than two) were not checked.

Thus, the current implementation only works with one local USB Bluetooth
device.

Change-Id: Ie84b02acbb2b7700016343438e2f068dd631497b
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>