RFR: 8340829: Generated API docs should clearly identify EA builds

[Kevin Rushforth]

This PR modifies the header and footer of the javadoc-generated API docs to add "DRAFT $VER-ea+$BLD" to clearly identify an ea build of the docs, and also to make it clear which build number the docs refer to. This matches was the JDK does. For GA builds, there will be no change to what we generate today. Since I had to add logic to filter the javadoc properties defined in `build.javadoc` to add the DRAFT message if not a GA build, I also changed the properties to not hard-code the major version of JavaFX, thus reducing the duplication.

I ran three builds to test this, an ordinary developer build, an EA build, and a GA build as follows:

#### Developer build

Command line:

gradle javadoc


Generated docs: https://cr.openjdk.org/~kcr/8340829/javadoc-internal/

#### EA build

Command line:

gradle -PPROMOTED_BUILD_NUMBER=98 -PHUDSON_BUILD_NUMBER=1234 -PHUDSON_JOB_NAME=jfx-dev javadoc


Generated docs: https://cr.openjdk.org/~kcr/8340829/javadoc-ea/

#### GA build

Command line:

gradle -PMILESTONE_FCS=true -PPROMOTED_BUILD_NUMBER=99 -PHUDSON_BUILD_NUMBER=4321 -PHUDSON_JOB_NAME=jfx-dev javadoc


Generated docs: https://cr.openjdk.org/~kcr/8340829/javadoc-fcs/

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Commit messages:
 - 8340829: Generated API docs should clearly identify EA builds

Changes: https://git.openjdk.org/jfx/pull/1583/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1583&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340829
  Stats: 19 lines in 3 files changed: 11 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1583.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1583/head:pull/1583

PR: https://git.openjdk.org/jfx/pull/1583