Handle text scaling for HiDPI environments/DPI changes

[compnerd]

Although views currently scale according to the DPI settings for the monitor, the text currently does not. The TraitCollection type has the necessary information which needs to be wired through to FontMetrics, FontDescriptor, and Font. The Window type conforms to TraitEnvironment and will send the traitCollectionDidChange(_:) which allows the responder chain to react and re-size fonts on views conforming to ContentSizeCategoryAdjusting.