RFR: 8244212: Optionally download media and webkit libraries from latest openjfx EA build

[Jesper Skov]

I have tested on Linux (Fedora 31) only.
It works as intended (with one test failure due to 15-ea+4 being too old now).

UPDATE_STUB_CACHE suggests there is some magic available to help manage the stub cache.
But as I could not find anything about it, that section in the documentation may need some love.

-------------

Commit messages:
 - Add build tooling note
 - Downloaded from MavenCentral note in build file
 - Review fixes
 - Allow use of official WebKit/Media libraries for stubs

Changes: https://git.openjdk.java.net/jfx/pull/202/files
 Webrev: https://webrevs.openjdk.java.net/jfx/202/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244212
  Stats: 106 lines in 2 files changed: 97 ins; 3 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/202.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/202/head:pull/202

PR: https://git.openjdk.java.net/jfx/pull/202