refactor: replace core directory to core library

[KazuCocoa]

• separate core as core ruby gem separate core as core ruby gem #665
• Add start/stop recording screen Add start/stop recording screen #674
• deprecate
  • ap_ prefixed logging
  • source_window: use get_source instead
  • ios_version: use platform_version instead
  • current_app: use current_activity, for example to know current app.