Add support for configurable fonts for window and tab titles. This is
only implemented for macOS (and could be macOS-only if other platforms
aren't able to support this using their windowing toolkits). It plays
nicely with regular and titlebar tabs.
We are also now using the `darken(by:)` implementation to generate
the split divider color on macOS, which means we'll have a consistent
rendering across iOS and macOS.
