[FIX] web_editor: prevent toolbar flicker on preview icon color change

If you clicked on a pictogram, then previewed various colors for it by
hovering colors in the colorpicker of the toolbar, the toolbar started
flickering uncomfortably. That's because the selection was changed
at every color change, which was unnecessary.

closes odoo/odoo#106788

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>