Replace arcade.Section with arcade.Scene

[ironkayman]

Sections are more input flow control oriented, while scenes are more about sprite(lists) interactions.

Stage 1

• [x] invasion view
  • [x] merge player and alien sections into a single scene
    • they remain separate, added mixins
  • [x] replace background
  • [x] replace overlay section with scenes
• [x] main menu view
  • [x] replace background section with same scene
  • [x] section with UI buttons stays the same? - yes, section inherits info about keyboard events throught veiw.

[ironkayman]

Also consider using https://github.com/maarten-dp/arcade-curtains as potentially more high-level abstraction of groups of sprites in a manager.