Locale-sensitive operations using `U16String` and common locale enumeration

[kugelbltz]

This PR works with PR #2773. Operations are prefixed with uni_

Case modification

uni_to_upper, uni_to_lower and uni_to_title operations using ICU operations.

String sorting

• uni_compare_to : unicode and locale aware string comparison using ICU's collator structure
• Array[String] sorter (StrSorter)

Formatting

Locale aware time/number to String formatting using ICU's number and time formatter structures

Locale

• The default locale is now a global variable (default_locale)
• Added some other common locales in Sys (could be unnecessary)