[code-reflection] Integrated: HAT simplify tool text styling

[Gary Frost]

Recent PR's have included components for styling Java and CodeReflection text.   

In preparation for allowing edits of text (for new upcomming tooling) I refactored the styling, so it can be reused.

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Commit messages:
 - Externalized TextModelViewer styling
 - Switching to external StyleMapper

Changes: https://git.openjdk.org/babylon/pull/514/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=514&range=00
  Stats: 902 lines in 12 files changed: 480 ins; 385 del; 37 mod
  Patch: https://git.openjdk.org/babylon/pull/514.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/514/head:pull/514

PR: https://git.openjdk.org/babylon/pull/514