Modernize font handling in Qt

There are a few areas where Qt has fallen behind on user expectations for font handling, and needs some effort.

1. OpenType feature selection: This has been available in CSS for quite some time, and is being requested by users.
2. Variable fonts: More and more font families are packed with multiple font styles / variations in a single file. While there might be some work/testing needed for system-installed fonts, that appears to work due to the system hiding the details of how fonts are actually stored, but for application fonts we do not support it at all, since we assume that applications font files correspond to a single font.
3. Emoji grapheme clusters: The font selection algorithm is having issues adapting to how emojis are done and is probably the area that needs the most work.