RFR: 8240692: Cleanup of the javafx property objects

[Nir Lisker]

A simple readability cleanup for the files that were changed in #113.

Note that the boolean property has an `instanceof` check that doesn't exist in the other properties.

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Commits:
 - 15f50f17: Initial commit

Changes: https://git.openjdk.java.net/jfx/pull/141/files
 Webrev: https://webrevs.openjdk.java.net/jfx/141/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240692
  Stats: 167 lines in 6 files changed: 12 ins; 124 del; 31 mod
  Patch: https://git.openjdk.java.net/jfx/pull/141.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/141/head:pull/141

PR: https://git.openjdk.java.net/jfx/pull/141