[FIX] web: visible studio hooks in listview

Before this commit, there are no hooks when dragging a field over an
editable listview. This was caused by wrong computation of column
width in studio mode.

This commit isolate the computation of column width, so that studio
can override it and make the correct computation.

Task-Id 2057829

closes odoo/odoo#36959

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>
Co-authored-by: Mohammed Shekha <msh@odoo.com>