wasapi: support Exclusive mode on UWP (use new function PaWasapi_SetDefaultInterfaceI...

wasapi: support Exclusive mode on UWP (use new function PaWasapi_SetDefaultInterfaceId to set device's interface id to make Exclusive mode work);
 - support for explicit 8.24 format (24-bit inside 32-bit container) by specifying paCustomFormat (see IMPORTANT notes in the header file);
 - new flag paWinWasapiExplicitSampleFormat to force explicit format and avoid fallback to a closest working;
 - new function PaWasapi_GetDeviceCurrentFormat to get device format of the opened stream;
 - support PaWasapi_ThreadPriorityXXX API on UWP to boot thread priority;
 - improved selection of the closest format in Exclusive mode.