RFR: 8320912: IME should commit on focus change

[Martin Fox]

This is a Mac only bug. If the user was in the middle of IM text composition and clicked on a different node the partially composed text was left in the old node and the IM window wasn't dismissed. This PR implements the existing finishInputMethodComposition call so it can commit the text and dismiss the IM window before focus moves away from the node where composition was taking place.

This PR changes the implementation of `unmarkText` to match what we want and what Apple says it should do ("The text view should accept the marked text as if it had been inserted normally"). With that said I haven't found an IME that calls this routine.

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Commit messages:
 - Proper IM window cleanup when focus moves from one node to another

Changes: https://git.openjdk.org/jfx/pull/1356/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1356&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320912
  Stats: 40 lines in 4 files changed: 36 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1356.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1356/head:pull/1356

PR: https://git.openjdk.org/jfx/pull/1356