It should be possible to customize the icon of a button in a tree view
Although it works on the client side, when trying to install the module the following error message is raised:
:9:0:ERROR:RELAXNGV:RELAXNG_ERR_INVALIDATTR: Invalid attribute icon for element button
<?xml version="1.0"?>
<!-- This file is part of Tryton. The COPYRIGHT file at the top level of
this repository contains the full copyright notices and license terms. -->
<tree>
icon="tryton-go-next"/>
</tree>
ERROR:convert:Error while parsing xml file:
The following review fixes it.