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.

Help/manual/cmake-properties.7.rst

@@ -254,17 +254,22 @@ Properties on Targets
    /prop_tgt/TYPE
    /prop_tgt/VERSION
    /prop_tgt/VISIBILITY_INLINES_HIDDEN
+   /prop_tgt/VS_DESKTOP_EXTENSIONS_VERSION
    /prop_tgt/VS_DOTNET_REFERENCES
    /prop_tgt/VS_DOTNET_TARGET_FRAMEWORK_VERSION
    /prop_tgt/VS_GLOBAL_KEYWORD
    /prop_tgt/VS_GLOBAL_PROJECT_TYPES
    /prop_tgt/VS_GLOBAL_ROOTNAMESPACE
    /prop_tgt/VS_GLOBAL_variable
+   /prop_tgt/VS_IOT_EXTENSIONS_VERSION
+   /prop_tgt/VS_IOT_STARTUP_TASK
    /prop_tgt/VS_KEYWORD
+   /prop_tgt/VS_MOBILE_EXTENSIONS_VERSION
    /prop_tgt/VS_SCC_AUXPATH
    /prop_tgt/VS_SCC_LOCALPATH
    /prop_tgt/VS_SCC_PROJECTNAME
    /prop_tgt/VS_SCC_PROVIDER
+   /prop_tgt/VS_WINDOWS_TARGET_PLATFORM_MIN_VERSION
    /prop_tgt/VS_WINRT_COMPONENT
    /prop_tgt/VS_WINRT_EXTENSIONS
    /prop_tgt/VS_WINRT_REFERENCES

Help/prop_tgt/VS_DESKTOP_EXTENSIONS_VERSION.rst

@@ -0,0 +1,10 @@
+VS_DESKTOP_EXTENSIONS_VERSION
+-----------------------------
+
+Visual Studio Windows 10 Desktop Extensions Version
+
+Specifies the version of the Desktop Extensions that should be included in the
+target. For example ``10.0.10240.0``. If the value is not specified, the Desktop
+Extensions will not be included. To use the same version of the extensions as
+the Windows 10 SDK that is being used, you can use the
+:variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION` variable.

Help/prop_tgt/VS_IOT_EXTENSIONS_VERSION.rst

@@ -0,0 +1,10 @@
+VS_IOT_EXTENSIONS_VERSION
+-------------------------
+
+Visual Studio Windows 10 IoT Extensions Version
+
+Specifies the version of the IoT Extensions that should be included in the
+target. For example ``10.0.10240.0``. If the value is not specified, the IoT
+Extensions will not be included. To use the same version of the extensions as
+the Windows 10 SDK that is being used, you can use the
+:variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION` variable.

Help/prop_tgt/VS_IOT_STARTUP_TASK.rst

@@ -0,0 +1,6 @@
+VS_IOT_STARTUP_TASK
+-------------------
+
+Visual Studio Windows 10 IoT Continuous Background Task
+
+Specifies that the target should be compiled as a Continuous Background Task library.

Help/prop_tgt/VS_MOBILE_EXTENSIONS_VERSION.rst

@@ -0,0 +1,10 @@
+VS_MOBILE_EXTENSIONS_VERSION
+----------------------------
+
+Visual Studio Windows 10 Mobile Extensions Version
+
+Specifies the version of the Mobile Extensions that should be included in the
+target. For example ``10.0.10240.0``. If the value is not specified, the Mobile
+Extensions will not be included. To use the same version of the extensions as
+the Windows 10 SDK that is being used, you can use the
+:variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION` variable.

Help/prop_tgt/VS_WINDOWS_TARGET_PLATFORM_MIN_VERSION.rst

@@ -0,0 +1,10 @@
+VS_WINDOWS_TARGET_PLATFORM_MIN_VERSION
+--------------------------------------
+
+Visual Studio Windows Target Platform Minimum Version
+
+For Windows 10. Specifies the minimum version of the OS that is being
+targeted. For example ``10.0.10240.0``. If the value is not specified, the
+value of :variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION` will be used on
+WindowsStore projects otherwise the target platform minimum version will not
+be specified for the project.