RFR: 8345063: Centralize version number of the Gradle build file

[Nir Lisker]

Centralizes version numbers in a single place. This is not the best way of doing it in Gradle, but it's an improvement that will simplify other changes. I used the `build.properties` file because other versions are already written there.

There are other version numbers in the build file, but they are not attached directly to a dependency. Mostly libav ones. I decided not to touch them.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jfx/pull/1649/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1649&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345063
  Stats: 44 lines in 2 files changed: 15 ins; 3 del; 26 mod
  Patch: https://git.openjdk.org/jfx/pull/1649.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1649/head:pull/1649

PR: https://git.openjdk.org/jfx/pull/1649