Enable the use of the Win32 Bluetooth backend

This change enables the optional use of the Win32-based Bluetooth
backend on Windows. By default, the WinRT backend is used, if
supported by the platform. The use of the Win32 backend must be
selected by the -native-win32-bluetooth configuration option.

Task-number: QTBUG-40698
Change-Id: I6904bf077467d826e3ff5ad026ebae5f955f2e37
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>