RFR: 8296266: TextArea: Navigation breaks with RTL text

[Andy Goryachev]

The fix uses character BreakIterator instead of the logic that relies on caretBounds/hitTest/rangeShape in TextInputControl.nextCharacterVisually().

I believe this is a more reliable method of navigation, as it behaves in sync with the jdk break iterator, thought it might work differently around grapheme clusters, considering a recent change JDK-8291660

This change also introduces TextInputControlHelper class (impl. detail) which gives access to character- and word- break iterators cached by TextInputControl (*some say* these iterators and associated editing logic should be a part of Content implementation, but that's a discussion for another day).

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Commit messages:
 - added robot test
 - exposing cached break iterators
 - using break iterator

Changes: https://git.openjdk.org/jfx/pull/1220/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1220&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296266
  Stats: 347 lines in 4 files changed: 255 ins; 60 del; 32 mod
  Patch: https://git.openjdk.org/jfx/pull/1220.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1220/head:pull/1220

PR: https://git.openjdk.org/jfx/pull/1220