RFR: 8276124: Provide snippet support for properties files

[Pavel Rappo]

The initial integration (JDK-8266666) of JEP 413 did not support properties files. This commit rights that wrong.

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Depends on: https://git.openjdk.java.net/jdk/pull/6359

Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6397/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6397&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276124
  Stats: 332 lines in 5 files changed: 290 ins; 26 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6397.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6397/head:pull/6397

PR: https://git.openjdk.java.net/jdk/pull/6397