Add ICU to handle Unicode text

[SamVanheer]

Add the ICU library to handle Unicode text.

Helper functions used to convert strings to upper- and lowercase in string_utils.h/.cpp need to be updated to use this library.

[SamVanheer]

Possible solution: https://github.com/unicode-org/icu https://github.com/viaduck/icu-cmake

The above project relies on some third party server that hosts pre-packed ICU builds of some kind so an alternative that doesn't rely on that would be preferable.