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

[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.

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Commit messages:
 - JDK-8302355: Public API for Toolkit.canStartNestedEventLoop()

Changes: https://git.openjdk.org/jfx/pull/1031/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1031&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302355
  Stats: 13 lines in 1 file changed: 12 ins; 0 del; 1 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