RFR: JDK-8302355: Public API for Toolkit.canStartNestedEventLoop() [v2]

[Marius Hanl]

> This PR adds the last missing method for dealing with nested event loops.
> As also written in the ticket, there is currently no (public) way to find out whether it is safe to start an event loop now or not.
> Classes like `Dialog` check via `Toolkit.getToolkit().canStartNestedEventLoop()` (sun api), if it safe to start a nested event loop before doing so.

Marius Hanl has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8302355: checkFxUserThread(), adjusted javadoc accordingly. Added Tests

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1031/files
  - new: https://git.openjdk.org/jfx/pull/1031/files/39f78715..36adae32

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1031&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1031&range=00-01

  Stats: 146 lines in 4 files changed: 140 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1031.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1031/head:pull/1031

PR: https://git.openjdk.org/jfx/pull/1031