Allow importing FXML files with javascript

[jperedadnr]

Issue

Fixes #794, by adding flag that enables reading the FXML file with javascript language. Also reverts a small change done in #619, as now the test as it was before passes.

Important note: Scene Builder (workspace/preview) does not evaluate the onAction eventHandlers, so there is no need for a Nashorn engine, and there is no risk of opening any FXML file with any kind of malicious script.

Progress

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