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).
By contract the first argument of the `fields_get` method is supposed to
be a list. Before this fix, `web_timeline` would call `fields_get` with
a string instead of a list.
Fortunately in case only 1 field is being grouped, this worked as Odoo
does an `x in y` comparison in its `fields_get` implementation, which
does pass in a simple `"project_id" in "project_id"` case.
But that call remains invalid and can break when `fields_get` has been
tweaked by other modules.
