RFR: 8307363: TextFlow.underlineShape()

[Andy Goryachev]

Adding TextFlow.underlineShape() to add support for a spellchecker-like decoration, using 

getRange(start, end, TextLayout.TYPE_UNDERLINE);


which mirrors an existing method in Text with exactly the same signature.

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Commit messages:
 - use the same javadoc as in Text
 - whitespace
 - 8307363: text flow underline shape

Changes: https://git.openjdk.org/jfx/pull/1127/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1127&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307363
  Stats: 63 lines in 2 files changed: 52 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1127.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1127/head:pull/1127

PR: https://git.openjdk.org/jfx/pull/1127