The OASIS spec [SAML V2.0 Subject Identifier Attributes Profile](https://wiki.oasis-open.org/security/SAMLSubjectIDAttr) defines two new standard attributes intending to replace use of persistent NameIDs and the eduPersonTargetedID and eduPersonUniqueId SAML attributes. The updated Kantara [SAML V2.0 Interoperability Deployment Profile](https://kantarainitiative.github.io/SAMLprofiles/saml2int.html) will also standardize on these new attributes (cf. SDP-SP15 ibid.).

This commit adds the two new attributes:

* to the URN attribute maps
* to attribute definitions and translations of the locale system
* to the smartattributes:SmartID auth proc filter (after any other SAML attributes, but before the non-SAML ones).

Support for saml2int SDP-SP16 (attribute requirements signalling via Entity Attributes) is not included here.

This allows us to use this new filter to translate strings from a given array of translations, where every translation is indexed by its ISO 639 code. A new configuration option ('language' -> 'priorities') is available too to control the alternative languages that can be used instead of a given language, when the latter is not found. The filter returns null when no suitable translation is found, so that it can be combined with "default()" to set a default translation for a given string.

Add a couple of methods to perform translations with PHP's native gettext implementation, and start using them.

These two methods are equivalent to gettext/gettext's __() and n__(), but using strtr() instead of vsprintf(), so that the placeholders can be names instead of printf() format strings.

This makes instanciating the Translate/Template-class unnecessary.

For #455

For #455

Both have been migrated to use namespaces.

More bugfixes. The SimpleSAML\Locale\Language::$languageParameterName and SimpleSAML\Locale\Language::$language properties should be private, and therefore not accessed from the outside of their respective classes. Add getters instead.

Rename \SimpleSAML\Locale\Translate::t_not_translated() to the more concise (and camel caps) getStringNotTranslated().

Return lang_merge() to SimpleSAML_XHTML_Template, as it was public, and mark it as deprecated. Use array_merge_recursive() instead.

Rename \SimpleSAML\Locale\Translate::getTranslation() to the more comprehensive \SimpleSAML\Locale\Translate::getPreferredTranslation().

A template now has a translator-object, SimpleSAML_Locale_Translate.
This handles the existing translation system.

The translator has a language-object, SimpleSAML_Locale_Language. This
handles discovery and switching of locale.

SimpleSAML_XHTML_Template has several methods that are wrapped
translator-methods:

t()
isLanguageRTL()
getLanguageList()
getTranslation()

This way it isn't necessary to change much of the existing system.

(being respectful with occurences that might change the behaviour, i.e. default database prefixes)

Harbinger of things to come...

Move SimpleSAML_Utilities:: setCookie() to SimpleSAML\Utils\HTTP:: setCookie() and deprecate the former.

Move SimpleSAML_Utilities:::getAcceptLanguage() to SimpleSAML\Utils\HTTP::getAcceptLanguage() and deprecate the former.