Implement detection of emoji sequences

Unicode has a concept of emoji presentation sequences: https://www.unicode.org/reports/tr51/#def_emoji_presentation_sequence

This includes:

• Characters that are predefined as emojis by the Unicode data,
• Flag sequences
• Tag sequences
• Keycap sequences
• ZWJ sequences

The shaper can handle clustering and glyph selection here, but we need to make sure the correct font is applied to the entire sequence. Therefore, we have to predetect these sequences and make sure that we select a color font for them (either the current font or a system default color font).

Hopefully, we can do this by parsing the strings and detecting these sequences ahead of time, and put these in separate QScriptItems and then in the font matcher we need to prefer emoji fonts for these.