Make LLTransUtil::parseStrings() merge all relevant strings.xml files.

Until now, adding a xui/en/strings.xml file in any non-default skin meant you
had to clone the entire file, editing only the particular entries you wanted
to override. With this change, we load strings.xml file(s) from the default
skin before loading the specified skin -- so a non-default skin can now
provide a strings.xml file containing only the specific entries it wants to
override.