- Changes internal target_os flags to be feature flags; macOS is now
appkit, and iOS/tvOS are now uikit. This enables platforms that are
not Apple-specific platforms that use frameworks to be compiled for.
- Updates the examples to handle closing/quitting better.
- Corrects `feature` -> `target_os` checks.
- Updates the old iOS scene delegate pieces to use the new class
structure.
- Bundles in an iOS demo app.
- Blocks off most things that should not even attempt to compile for
iOS.
