Upgrade to work with Bevy 0.15 from Bevy 0.12

[gazliddon]

Migration guide: 0.12 to 0.15 (Bevy 0.12 to 0.15)

There were a lot of API changes from 0.12 to 0.15 including numerous
trait method name changes and some functional changes as well.

• Removed required-features from examples in Cargo.toml
• Replaced deprecated push_children with add_children
• Changes to work with method changes in CameraProjection
• Changed to work with API renames in VisibilitySystems
• ScalingMode::WindowSize changes, zoom no longer held in Enum
• Flappin Example
  • Inserted gamestate as a resource
  • close_on_esc deprecated, added own implementation
  • Arguments to TextureAtlasLayout::from_grid updated
  • Replaced deprecated Camera2dBundle with Camera2d component
  • Removed gamepad code
    • 0.15 crashes if no gamepad is plugged in
  • Removed queries for TextureAtlas components
    • TextureAtlas are now optional members of Sprite
• Mire Example
  • Updated for 0.15
• Added readme example
  • Duplicate of example code in this file