I created a new model called res.partner.link.category. Then, I created a button which opens a customized tree of that model. Here is the function called by the button and which returns the tree:

def open_history(self, cr, uid, ids, context=None):
    rplc_obj = self.browse(cr, uid, ids[0], context=context)
    data_obj = self.pool.get('ir.model.data')
    tree_data_id = data_obj._get_id(cr, uid, 'res_partner_extended', 'res_partner_link_category_tree_view')
    form_data_id = data_obj._get_id(cr, uid, 'res_partner_extended', 'res_partner_link_category_edit_form_view')
    tree_view_id = False
    form_view_id = False
    if tree_data_id:
        tree_view_id = data_obj.browse(cr, uid, tree_data_id, context=context).res_id
    if form_data_id:
        form_view_id = data_obj.browse(cr, uid, form_data_id, context=context).res_id
    return {
        'name': 'History of "%s"' % rplc_obj.link_category_id.name,
        'view_type': 'form',
        'view_mode': 'tree',
        'view_id': False,
        'views': [(tree_view_id, 'tree'), (form_view_id, 'form'),],
        'res_model': 'res.partner.link.category',
        'domain': '[("partner_id", "=", %d), ("link_category_id", "=", %d)]' % (rplc_obj.partner_id, rplc_obj.link_category_id.id),
        'type': 'ir.actions.act_window',
        'nodestroy': True,
        'target': 'new',
        'flags': {'tree': {'action_buttons': True},
                  'form': {'action_buttons': True},}
    }

And this is the XML of the tree which is being shown:

<record id="res_partner_link_category_tree_view" model="ir.ui.view">
    <field name="name">res.partner.link.category.tree</field>
    <field name="model">res.partner.link.category</field>   
    <field name="type">tree</field>
    <field name="arch" type="xml">
        <tree string="Link categories">
            <field name="type"/>
            <field name="date"/>
            <field name="observations"/>
        </tree>
    </field>
</record>

Everything is OK, except for the lack of delete buttons in the tree. I need them, why can't I see them? Is there any option or flag (like action_buttons) which must be specified? How can I show them?