RFR: 8349096: Split/MenuButton: exception initializing in a background thread

[Andy Goryachev]

## Root Cause

The ContextMenu (PopupWindow) cannot be shown in a background thread.

## Solution

Bail out of `show()` if in a background thread.

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Commit messages:
 - split / menu button fix

Changes: https://git.openjdk.org/jfx/pull/1709/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1709&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349096
  Stats: 13 lines in 2 files changed: 4 ins; 5 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1709.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1709/head:pull/1709

PR: https://git.openjdk.org/jfx/pull/1709