RFR: 8322703: Intermittent crash in WebView in a JFXPanel from IME calls on macOS

[Kevin Rushforth]

As noted in the JBS bug, this is a follow-on to [JDK-8196011](https://bugs.openjdk.org/browse/JDK-8196011) that I discovered while testing the fix for [JDK-8221261](https://bugs.openjdk.org/browse/JDK-8221261) (a deadlock in the IME code when using WebView in a JFXPanel on macOS).

I have tested this in connection with with the proposed fix for JDK-8221261, although it is a valid fix regardless.

This expands the fix done in [JDK-8221261](https://bugs.openjdk.org/browse/JDK-8221261) to call all of the WebKit methods on the right thread. Additionally, we sometimes see spurious exceptions where the committed text is coming back as null, so I changed the log level to "fine" rather than "severe" for those exceptions. I'll file a follow-up bug  to see if any of these are real problems or not.

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Commit messages:
 - 8322703: Intermittent crash in WebView in a JFXPanel from IME calls on macOS

Changes: https://git.openjdk.org/jfx/pull/1321/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1321&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322703
  Stats: 62 lines in 1 file changed: 33 ins; 18 del; 11 mod
  Patch: https://git.openjdk.org/jfx/pull/1321.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1321/head:pull/1321

PR: https://git.openjdk.org/jfx/pull/1321