RFR: 8249777: build.gradle: project.version should not contain time stamps

[Kevin Rushforth]

The addMavenPublication method sets the `project.version` to `$MAVEN_VERSION`, where project is base, graphics,
controls, etc. When doing an ordinary "developer" build, this property contains the time stamp . This is causing
problems with the NetBeans gradle plugin not being able to determine the dependencies between the projects (modules).

We haven't noticed this bug before now, because we suppress the creation of the individual project jar files, but if we
were creating them, we would have the problem that each time we ran gradle, it would generate a new
`project-$version.jar` file with a different time stamp each time the build was run.

The setting of `project.version=$MAVEN_VERSION` is only used by the maven "publish" tasks, which isn't done for
developer builds nor for most productions builds.

The proposed solution is to add a new boolean `MAVEN_PUBLISH` flag, which will default to `false`. This flag will
qualify the setting of the `MAVEN_VERSION` property and the configuration of the maven publishing tasks. After this
change, it will be necessary to run gradle with `-PMAVEN_PUBLISH=true` in order to run the maven publishing tasks.

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Commit messages:
 - 8249777: build.gradle: project.version should not contain time stamps

Changes: https://git.openjdk.java.net/jfx/pull/270/files
 Webrev: https://webrevs.openjdk.java.net/jfx/270/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8249777
  Stats: 11 lines in 1 file changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/270.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/270/head:pull/270

PR: https://git.openjdk.java.net/jfx/pull/270