Change chat menu button to three vertical dots

briar-gtk/data/ui/main_window.ui

@@ -372,7 +372,7 @@
                       <object class="GtkImage">
                         <property name="visible">True</property>
                         <property name="can_focus">False</property>
-                        <property name="icon_name">open-menu-symbolic</property>
+                        <property name="icon_name">view-more-symbolic</property>
                       </object>
                     </child>
                     <accessibility>