Support for Windows OneCore voices included in Windows 10.
This uses a C++/CX dll to access the UWP SpeechSynthesizer class. There are other UWP APIs we might like to access in future (e.g. OCR), so rather than making this dll specific to OneCore speech, it's called nvdaHelperLocalWin10. The build system for this dll makes it easy to add other components in future.
In addition, this required code to generate balanced XML from an NVDA speech sequence. Although we use SSML for eSpeak, eSpeak happily accepts unbalanced (malformed) XML. OneCore speech does not. This code is in the speechXml module. This might eventually be reused to replace the ugly balanced XML code in the SAPI5 driver.
