RFR: 8345063: Centralize version number of the Gradle build file [v2]

[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.

Nir Lisker has updated the pull request incrementally with one additional commit since the last revision:

  Added EOF line

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1649/files
  - new: https://git.openjdk.org/jfx/pull/1649/files/15dd5a8f..df9ae398

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1649&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1649&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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