RFR: 8267554: Support loading stylesheets from data-URIs

[Michael Strauß]

This PR extends data URI support to allow stylesheets to be loaded from data URIs.

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Commit messages:
 - Stylesheets can be loaded from data URIs

Changes: https://git.openjdk.java.net/jfx/pull/536/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=536&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267554
  Stats: 237 lines in 4 files changed: 218 ins; 7 del; 12 mod
  Patch: https://git.openjdk.java.net/jfx/pull/536.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/536/head:pull/536

PR: https://git.openjdk.java.net/jfx/pull/536