Implement UTF-16 support

[Artoria2e5]

UTF-16 support -- support for surrogates -- is crucial for "modern" texts like Emoji and other new charcters. Microsoft Windows has supported UTF-16 since 2000. The Cairo backend currently used can too handle these characters.

[filipnavara]

The Cairo text rendering backend is broken pretty much for anything outside of the Unicode basic plane. It doesn't support RTL, BiDi or proper shaping, so UTF-16 is least of the concern. If you are aware of any other code that doesn't properly support UTF-16 we are likely to fix it. Otherwise you can weigh in on making Pango the default (or only) text rendering backend (#269).