[PATCH 0/1] MR2561: widl: Add support for composable attribute.

[Mohamad Al-Jaf (＠maljaf)]

Needed to build windows.ui.composition.idl.

Example:

`
    [
        composable(Windows.UI.Composition.ICompositionEasingFunctionFactory, public, Windows.Foundation.UniversalApiContract, 2.0),
        contract(Windows.Foundation.UniversalApiContract, 2.0),
        marshaling_behavior(agile),
        static(Windows.UI.Composition.ICompositionEasingFunctionStatics, Windows.Foundation.UniversalApiContract, 12.0),
        threading(both)
    ]
    runtimeclass CompositionEasingFunction : Windows.UI.Composition.CompositionObject
    {
        [default] interface Windows.UI.Composition.ICompositionEasingFunction;
    }
`

Some WinRT headers use `protected` rather than `public` for the composable attribute like windows.ui.xaml.media.idl:2304

`[composable(Windows.UI.Xaml.Media.ICacheModeFactory, protected, Windows.Foundation.UniversalApiContract, 1.0)]`