skiko is supposed to be a wrapper around skia, however it has unrelated
features like input. This PR removes it from this library - it's
supposed to be handled externally.

- Removed `SkikoInput`, `SkikoKey`, `SkikoPlatform*Event`
- Replaced `SkikoView` to `SkikoRenderDelegate` and related field in
`SkiaLayer`
- `SkiaLayer` on macOS native now receives `NSView` instead of creating
it internally and attaching it to the `NSWindow`. In theory, it allows
initializing non-full-window skia view there (not tested)
- Updated samples and readme