RFR: 8302472 WebColorFieldSkin should use precompiled Pattern

[John Hendrikx]

Split off from https://github.com/openjdk/jfx/pull/837

This uses a precompiled regex pattern and cleans up the code in WebColorFieldSkin a bit.

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Commit messages:
 - Use a precompiled Pattern and simplify code

Changes: https://git.openjdk.org/jfx/pull/1032/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1032&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302472
  Stats: 31 lines in 1 file changed: 7 ins; 13 del; 11 mod
  Patch: https://git.openjdk.org/jfx/pull/1032.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1032/head:pull/1032

PR: https://git.openjdk.org/jfx/pull/1032