RFR: 8220722: ProgressBarSkin: adds strong listener to control's width property

[Jeanette Winzenburg]

fix for https://bugs.openjdk.java.net/browse/JDK-8220722

- replaces the manually registered listener with registerChangeListener(...)
- added test that's failing before and passing after the fix (plus a sanity test that the skin still is listening to changes)

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Commits:
 - 9f4fc409: 8220722: ProgressBarSkin: adds strong listener to control's width
 - 61f7361d: 8235151: Nonexistent notifyQuit method referred from iOS GlassHelper.m

Changes: https://git.openjdk.java.net/jfx/pull/59/files
 Webrev: https://webrevs.openjdk.java.net/jfx/59/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8220722
  Stats: 102 lines in 5 files changed: 88 ins; 0 del; 14 mod
  Patch: https://git.openjdk.java.net/jfx/pull/59.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/59/head:pull/59

PR: https://git.openjdk.java.net/jfx/pull/59