RFR: 8276124: Provide snippet support for properties files [v4]

[Pavel Rappo]

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

Pavel Rappo has updated the pull request incrementally with one additional commit since the last revision:

  Address feedback

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6397/files
  - new: https://git.openjdk.java.net/jdk/pull/6397/files/bd9aadab..19e8db24

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6397&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6397&range=02-03

  Stats: 19 lines in 1 file changed: 6 ins; 11 del; 2 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