Allow deletion of timeline

This issue was already raised in #74 (closed), which has been closed with the merger request !59 (merged). The deletion is not yet fully implemented. For now, ony the annotations are removed from the timeline. This makes, de facto, the timeline disappears from the CSV file (unless new annotations are added before saving the CSV file or the project), but is it still visible and functional in the time pane.

We must add provisions for easily repositioning the timeline in the time pane and shrink the time pane accordingly. This will imply a substantial rewrite of the code of the TimeLineWidget class.