Make AbstractElement.USE_STRINGVALUE_SEPARATOR field value configurable

[dmak]

In my case there is a need to implement similar logic as in AbstractElement#getStringValue() namely separate string values of consequent elements with some separator (space or newline). This as to API this requires the introduction of new DocumentFactory instance that will produce "custom" DefaultElement instances. Quite some work. Alternatively I suggest to make this option configurable either by having a static setter, or via system property:

public abstract class AbstractElement ... {

    protected static final String STRINGVALUE_SEPARATOR = System.getProperty("org.dom4j.element.string_value_separator", null);
    ...
    public String getStringValue() {
        ...
        if (STRINGVALUE_SEPARATOR != null) {
            if (buffer.length() > 0) {
                buffer.append(STRINGVALUE_SEPARATOR);
            }
        }
    }
}