expression builder: disabled button when converter is not supported

fixes the display of the button being quirky
also fix the button vanish when value==False

views/ir_actions_report.xml

@@ -44,6 +44,28 @@
                                 <field name="keyword" />
                                 <field name="converter" />
                                 <field name="value" />
+                                <button
+                                    name="action_build_expression"
+                                    type="object"
+                                    string="Build expression"
+                                    class="oe_highlight"
+                                    invisible="not (value and value!='') or not (converter and converter in ['field','relation-to-many'])"
+                                    confirm="This will delete the current substitution value. Continue anyway ?"
+                                />
+                                <button
+                                    name="action_build_expression"
+                                    type="object"
+                                    string="Build expression"
+                                    class="oe_highlight"
+                                    invisible="(value and value !='') or not (converter and converter in ['field','relation-to-many'])"
+                                />
+                                <button
+                                    name="action_build_expression"
+                                    type="object"
+                                    string="Build expression"
+                                    class="oe_highlight disabled"
+                                    invisible="converter in ['field','relation-to-many']"
+                                />
                             </list>
                         </field>
                     </group>