Teach the VS 2015 generator to support WindowsStore 10.0 applications.
Add target properties to customize them:
* VS_WINDOWS_TARGET_PLATFORM_MIN_VERSION: Specifies the minimum version
of the OS that the project can target.
* VS_DESKTOP_EXTENSIONS_VERSION, VS_MOBILE_EXTENSIONS_VERSIONS,
VS_IOT_EXTENSIONS_VERSION: Add a reference to the version of the SDK
specified to the target allowing to target the extended functionality in
a universal project.
* VS_IOT_STARTUP_TASK: Specifies that the target should be
built as an IOT continuous background task.
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