feat: colour emojis without fontconfig hacks

Since getting Qt to resolve to a colour emoji font for emojis by (ab)using fontconfig often doesn't work and relies on the distro to ship a fontconfig hack that works with their configuration, this MR simply singles out emojis in text and sets their font to emoji; which resolves to the system's choice of emoji font, which is typically a colour emoji font.

Note that this adds a new dependency on ICU; though Qt uses it; so portability shouldn't be a problem.

Fixes #341.