RFR: 8264501: UIWebView for iOS is deprecated

[Jose Pereda]

This PR replaces the deprecated iOS native view for web (UIWebView) with WKWebView. 

While most of the native API can be easily replaced from one to the other, there are some changes that affect the scripts execution, and therefore some minor changes in WebEngine/JS2JavaBridge are required.

There is also a side effect if named members of JS objects are used via JSObject::setMember. The Java callbacks to JavaScript are now async, and the return value will be ignored. A workaround is to use JSObject::call instead to pass that value. This should be dealt with in a follow-up issue.

Also, a peer is provided to allow having a JavaFX node that can be resized and relocated.

This PR has been tested on iOS, loading URLs and local html files with JS scripts.

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Commit messages:
 - Replace UIWebView with WKWebView

Changes: https://git.openjdk.java.net/jfx/pull/449/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=449&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264501
  Stats: 264 lines in 9 files changed: 175 ins; 17 del; 72 mod
  Patch: https://git.openjdk.java.net/jfx/pull/449.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/449/head:pull/449

PR: https://git.openjdk.java.net/jfx/pull/449