By default, all btn colors are suggested in the link dialog (alpha,
beta, gamma, delta, epsilon, link, primary, secondary, success, info,
warning and danger). In lots of themes (and the default one), primary
= alpha and secondary = beta, which led to having two suggestion with
the same apparent color.
A new system was introduced to automatically remove the duplicates...
but this system also removed the button preview as it considered it to
be a duplicate.
This commit also adapts the preview to BS4.
task-1878245
qsm-odoo
39a441e902