Animation fadeheaderbehavior

[LanceMcCarthy]

Added FadeHeaderBehavior.cs to the Animations.Behaviors namespace. Added a FadeHeader demo to the Examples application.

[msftclas]

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[ScottIsAFool]

Remove usages of "this."

[msftclas]

@LanceMcCarthy, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, MSBOT;

[ScottIsAFool]

These two if statements can be merged and see whether AssociatedObject is ListViewBase which both ListView and GridView are. It's ListViewBase which has the Header property on it.

[ScottIsAFool]

LGTM

[deltakosh]

Do you mind updating the documentation as well?
https://github.com/Microsoft/UWPCommunityToolkit-docs

[LanceMcCarthy]

@deltakosh Almost done, have to add a new section because this behavior can't be added directly to the existing animations list

[deltakosh]

Understood!

[LanceMcCarthy]

Submitted PR for docs microsoft/UWPCommunityToolkit-docs#25

[deltakosh]

LGTM