RFR: 8252446: Screen.getScreens() is empty sometimes

[Kevin Rushforth]

As noted in the bug report, we get a pair of change events every time the list of screens changes. First, a change is
sent with an empty list of screens and then a change is sent with the new list of screens. This happens whenever a
monitor is plugged in or unplugged. It also happens on Mac at application startup.

As noted in the bug the reason for this is because the `updateConfiguration` method makes two separate calls on the
list of screens, `clear` and `addAll`, rather than calling `setAll`. The latter ensures that only a single change event
is delivered.

I verified that before this fix, the example program attached to the bug works correctly after the fix.

I wrote a unit test. It ends up being skipped on Windows and Linux since we don't get an initial change event. On Mac
the test fails without the fix and passes with the fix.

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Commit messages:
 - 8252446: Screen.getScreens() is empty sometimes

Changes: https://git.openjdk.java.net/jfx/pull/295/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=295&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252446
  Stats: 107 lines in 2 files changed: 103 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/295.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/295/head:pull/295

PR: https://git.openjdk.java.net/jfx/pull/295