[FIX] web_editor: make class-based colors selectable for custom buttons

Before this commit selecting a class-based color (gray scale,
o-color-1~5) was ignored when configuring a custom button colors.

After this commit the color's class is applied.
Additionally, upon switching between Custom and other types, the custom
UI is now cleaned up.

task-2653874

closes odoo/odoo#76936

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>