Make dirty field management more accurate

A document's property is now dirty only when the new value is different. The DublinCoreListener update the dc:modified, dc:contributors and dc:lastContributor on beforeDocumentModification only if the document is dirty (i.e. only if some value changed). The modified and contributors are still updated on lifecycle_transition_event even if the document is not dirty. There's a dirty management on array property items: ArrayProperty.isDirty(index). A document's snapshot version is created only if any field changed (doc is dirty). A method is available on property to force the dirty status even if the property value doesn't change on setValue (setForceDirty). It's used by the SimpleDocumentModel class to keep the previous behaviour on it. The DocumentValidationService validates only dirty properties if it's asked as usual - but also validates only dirty sub properties and dirty arrayproperty's items. Many test were updated (when a snapshot creation was expected without any change).