RFR: 8176270: Adding ChangeListener to TextField.selectedTextProperty causes StringOutOfBoundsException

[Oliver Kopp]

This is a WIP PR. Requesting for comments.

I could not replicate the test given at https://bugs.openjdk.java.net/browse/JDK-8176270 without TestFX. I nevertheless let my try to replicate the @Test things in here.

The fix is just a wild guess without really understanding the side effects of `.addListener`.

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Commits:
 - 492cb6ca: 8176270: Fix TextInputControl String boundaries

Changes: https://git.openjdk.java.net/jfx/pull/73/files
 Webrev: https://webrevs.openjdk.java.net/jfx/73/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8176270
  Stats: 108 lines in 2 files changed: 106 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/73.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/73/head:pull/73

PR: https://git.openjdk.java.net/jfx/pull/73