Breaking Changes
* remove `drag_context` property in https://github.com/kivy-garden/draggable/pull/17
* rename `on_drag_success` -> `on_drag_succeed` in https://github.com/kivy-garden/draggable/pull/18
* replace `DragContext.state` with `KXDraggableBehavior.drag_state` in https://github.com/kivy-garden/draggable/pull/19
* `None` is no longer a valid argument for `ongoing_drags()` in https://github.com/kivy-garden/draggable/pull/20
* rename `drag_start_from_others_touch()` -> `start_dragging_from_others_touch()` in https://github.com/kivy-garden/draggable/pull/21
* adapt to asynckivy 0.6.0 in https://github.com/kivy-garden/draggable/pull/26