WebView sources, both Java and native, can be open sourced now.

The plan is:
- Merge rt-closed/javafx-ui-webnode and rt-closed/webkit into rt/webview
- Move contents of the webnode/ repo to rt/webview/native
- Retire webnode/ repo

Changes to Ant scripts will be kept to a minimum. Gradle scripts will initially be updated just to have the build complete. Later they will be gradually modernized over time.