When an item label does not fit in its date-range box, overflow
according to
https://visjs.github.io/vis-timeline/examples/timeline/items/rangeOverflowItem.html
Previous CSS code was already trying to do that, but was selecting
`.vis-item.vis-item-content` instead of `.vis-item .vis-item-content`.
Displaying overflow text brings up layout issues solved by removing the
forced-100% width instruction.
This change also adds highlight when hovering a box, which is useful on
text that has overflown (as it has no borders).