VS: Add support for Windows 10 Universal (Store) Applications

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.