In grouped KanbanView, group's width follows three usecases:
- on smaller screens, the group (aka. column) takes 90% of the viewport
  (to give a hint of the next/previous column)
- on desktop-like screens, the group's width is based on the regular record's
  width and padding
- on desktop-like screens, a smaller variant of the one above (cf.
  `o_kanban_small_column`)

As the definition of those usecases where a bit
all-over-the-place, this commit takes advantage of the CSS custom
properties and the cascade to conditionally define the
`--KanbanGroup-width` property and set only once the group's width (cf.
`flex-basis`).

Note: it fixes a small issue where the "quick create" column was always
set to the smaller variant, even with regular record size.

Part-of: odoo/odoo#105531