[BEAM-1425] Window should comply with PTransform style guide

[jkff]

Incompatible changes:

• Window.Bound class is now simply Window: matters for users that were extracting the transform into a variable.
• Static methods such as Window.triggering(), Window.withAllowedLateness() etc. are now available via Window.configure() - e.g. Window.configure().withAllowedLateness(...). The method Window.into() is left intact as it is the primary entry point for windowing a collection, whereas the other methods are for adjusting the parameters of the windowing function.

R: @tgroh

[coveralls]

Changes Unknown when pulling 0ec3afc on jkff:window-style into ** 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/8924/
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[jkff]

Ping.

[tgroh]

LGTM