XMLSyntaxParser: tolerate a UTF-8 BOM at the start of XML documents

[NicolaIsotta]

XMLSyntaxParser rejected any leading text before the document element with "Invalid token ... found in document", so a BOM character (U+FEFF) caused every BOM-prefixed XML file to fail parsing. Treat a trimmed BOM the same as blank/whitespace when validating text under the Document node.

This also adds a test with a BOM xml file and removes some dead code (tokenSequence.token() is always null before the first moveNext())

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[NicolaIsotta]

NB: I discovered this bug while working on a JSF project whose faces-config.xml had the BOM.
Got an exception and autocomplete on the facelets stopped working.
Here's the stack trace:

java.lang.NullPointerException: Cannot invoke "org.w3c.dom.Document.getDocumentElement()" because the return value of "org.netbeans.modules.web.jsf.api.facesmodel.JSFConfigModel.getDocument()" is null
	at org.netbeans.modules.web.jsf.impl.metamodel.JsfModelImpl.createModel(JsfModelImpl.java:303)
	at org.netbeans.modules.web.jsf.impl.metamodel.JsfModelImpl.initModels(JsfModelImpl.java:333)
	at org.netbeans.modules.web.jsf.impl.metamodel.JsfModelImpl.<init>(JsfModelImpl.java:94)
	at org.netbeans.modules.web.jsf.impl.metamodel.JsfModelImplementation.<init>(JsfModelImplementation.java:44)
	at org.netbeans.modules.web.jsf.impl.metamodel.JsfModelImplementation.create(JsfModelImplementation.java:50)
	at org.netbeans.modules.web.jsf.impl.metamodel.JsfModelProviderImpl.createMetaModel(JsfModelProviderImpl.java:76)
	at org.netbeans.modules.web.jsf.impl.metamodel.JsfModelProviderImpl.getModel(JsfModelProviderImpl.java:69)
	at org.netbeans.modules.web.jsf.JSFUtils.getModel(JSFUtils.java:425)
	at org.netbeans.modules.web.jsf.api.editor.JsfFacesComponentsProvider.getLibraries(JsfFacesComponentsProvider.java:74)
	at org.netbeans.modules.web.jsf.editor.facelets.FaceletsLibrarySupport$RefreshFacesComponentsTask.run(FaceletsLibrarySupport.java:451)
	at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1370)
	at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:45)
	at org.openide.util.lookup.Lookups.executeWith(Lookups.java:287)
[catch] at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:1981)