RFR: 8274817: Allow JavacFiler subclasses and register them in Context

[Jan Lahoda]

An alternative to https://github.com/openjdk/jdk/pull/5076 - this allows to subclass `JavacFiler` and install it to the `Context`. The subclass then can handle the originating elements as needed.

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Commit messages:
 - Removing trailing whitespace
 - Updating test.
 - Merge branch 'master' into javacfiler-is-a-service
 - Making JavacFiler a Context service

Changes: https://git.openjdk.java.net/jdk/pull/5840/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5840&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274817
  Stats: 297 lines in 3 files changed: 295 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5840.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5840/head:pull/5840

PR: https://git.openjdk.java.net/jdk/pull/5840