[BEAM-1645] Populate display data on Window.Assign

[kennknowles]

Be sure to do all of the following to help us incorporate your contribution
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  [BEAM-<Jira issue #>] Description of pull request
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Do not review yet; running through Jenkins while checking other things.

[kennknowles]

It is actually much better to keep the diff behavior we have now. We can just reify the diff, and we have uses for that elsewhere too.

[coveralls]

Coverage increased (+0.01%) to 70.055% when pulling b716615 on kennknowles:Assign-DisplayData into 4c3fce1 on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/8184/

Failed Tests: 2

beam_PreCommit_Java_MavenInstall/org.apache.beam:beam-sdks-java-core: 2

• org.apache.beam.sdk.transforms.windowing.WindowTest.testDisplayDataExcludesUnspecifiedProperties
• org.apache.beam.sdk.transforms.windowing.WindowTest.testDisplayData

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