Support adding a 3D app launcher for when the app is used in MR

[mrlacey]

For new Features:

Overview:
When an app is placed in the MR Cliff House it just appears as a flat tile, but it is also possible for apps to provide a 3D laucher so an app can appear to have a more natural, integrated experience.

Add a feature to include a default 3D app launcher model in the project and make relevant associated manifest changes.
There will be no impact on the generated app if it is used on a normal desktop or any other device family.
Adding this feature will make it easier to add the 3D launcher to any app and hopefully encourage more people to think about how they can provide the best app experience when used in a mixed reality environment.

Links:

• https://docs.microsoft.com/en-us/windows/mixed-reality/3d-app-launcher-design-guidance
  https://docs.microsoft.com/en-us/windows/mixed-reality/creating-3d-models-for-use-in-the-windows-mixed-reality-home
• https://github.com/Microsoft/glTF-Toolkit (for converting the GLB files appropriately)
• https://github.com/Microsoft/Windows-universal-samples/tree/master/Samples/MixedRealityModel

Will only be possible once FCU is our min supported platform

[mrlacey]

I think a 3D version of the WTS logo would make a good default/placeholder option unless anyone has a better suggestion.

(I was bored last night)

[mrlacey]

Hey! What's that on the floor?

Lets launch a standard app

launching from a 3D object is much simpler.

Plus it alllows you to decorate your house ;)

[mrlacey]

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