Draft: general: Initial port to Rust

This is an initial port of Fonts to Rust. The port doesn't do much - it just loads a list of fonts and displays it. There's a lot of work to do for feature parity with the C version:

• Fonts should be loaded on another thread
• Font loading is off. Fonts have different names - though the algorithm should be roughly the same - and a different amount is loaded. I need to research font crates.
• Related, font-kit is not actively maintained. We should not rely on it.
• The font preview widget needs a rework, preferrably something based on the font manager mockups.
• Implement font installation
• Implement font details
• Implement thumbnailer
• Listen for changes to fontconfig and reload fonts

The C files mention sushi, but we don't need to implement anything for that. Sushi has moved its font preview code in-tree.