- Implement a more efficient way of refreshing timeline after a record
  update;
- Make `attrs` attribute work;
- When grouping by m2m and more than one record is set, the timeline
  item appears only on one group. Allow showing in both groups.
- When grouping by m2m and dragging for changing the time or the group,
  the changes on the group will not be set, because it could make
  disappear the records not related with the changes that we want to
  make. When the item is showed in all groups change the value according
  the group of the dragged item.
- When an item label does not fit in its date-range box: ✅ the label
  correctly overflows the box; ✅ clicking anywhere on the label allows
  moving the box; ❌ double-clicking the label outside of the box does
  not open that item.
