Publish DragEvent::Canceled always when picked Pane is dropped outside valid target area

[jsatka]

Currently a pane_grid::DragEvent::Canceled is not published when a picked pane_grid::Pane is dropped near the origin point where it was picked up (closer than DRAG_DEADBAND_DISTANCE) or outside the window. However in both cases in the pane grid state pane_grid::Memory the action state gets set from Action::Dragging to Action::Idle i.e. cancelling the dragging action.

As such, clicking a pane title bar produces DragEvent::Picked event but not subsequent DragEvent::Canceled, leaving a user who needs to track the dragging state thinking the pane is being dragged while it is not.

This change adds publishing Canceled variant in the aforementioned cases. Thus, dropping a picked pane will always produce either Dropped or Canceled variant depending whether the drop point was a valid pane_grid:::Target area.

[jsatka]

Easy way to test this is to run the pane_grid example with the following changes to Message handling to print all drag events to console:

            Message::Dragged(pane_grid::DragEvent::Picked { pane }) => {
                println!("{:?} drag started", pane);
            }
            Message::Dragged(pane_grid::DragEvent::Dropped {
                pane,
                target,
            }) => {
                self.panes.drop(pane, target);
                println!("{:?} drag dropped", pane);
            }
            Message::Dragged(pane_grid::DragEvent::Canceled { pane }) => {
                println!("{:?} drag canceled", pane);
            }

and then testing:

• clicking the pane title bars
• dragging and dropping panes outside the window