implement/overlay_manager.impl library

1. flutter_overlay_manager package
2. documentation
3. implement/overlay_manager.impl.dart

flutter_overlay_manager package

1. Libraries
2. flutter_overlay_manager
3. implement/overlay_manager.impl
4. mixin/overlay_mixin
5. model/loader
6. model/loading_payload
7. model/overlay_data
8. model/overlay_position
9. repository/overlay_manager
10. widget/dot
11. widget/four_rotating_dots
12. widget/overlay_widget