RFR: 8349105: Pagination: exception initializing in a background thread [v3]

[Andy Goryachev]

> ## Root Cause
> Animation gets started in a background thread, which causes the animation handler to run in the FX application thread, thus creating simultaneous access to the control's fields (list of children in this case).
> 
> ## Solution
> Postpone the animation unless running in the FX application thread.  There is no functional difference if the component is created/used in the FX application thread.

Andy Goryachev has updated the pull request incrementally with one additional commit since the last revision:

  gc

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1698/files
  - new: https://git.openjdk.org/jfx/pull/1698/files/a3330b2d..300b269d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1698&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1698&range=01-02

  Stats: 6 lines in 1 file changed: 4 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1698.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1698/head:pull/1698

PR: https://git.openjdk.org/jfx/pull/1698