GEOT-4477 introduced support for internationalized strings in SLD Title and Abstract elements

Changed Title and Abstract elements in both main gt-styling and gt-xsd-sld modules to support mixed content.
Using Localized elements it's now possibile to specify alternative values in different locales.

Example.
<Localized lang="it">italian</Localized>

To extract localized values you can use for example rule.getDescription().getTitle().toString(Locale.ITALIAN)