RFR: 8349105: Pagination: exception initializing in a background thread [v5]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge branch '8349756.pagination.leak' into 8349105.thread.safety.pagination
 - back to constructor
 - memory leak
 - remove gc
 - slow down gc
 - gc
 - Merge branch 'master' into 8349105.thread.safety.pagination
 - postpone animation

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1698/files
  - new: https://git.openjdk.org/jfx/pull/1698/files/3256957e..eba7b350

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1698&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1698&range=03-04

  Stats: 218 lines in 43 files changed: 94 ins; 38 del; 86 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