[FIX] point_of_sale: allow opening of sessions with cash control enabled

Without cash control the pos_session workflow automatically sets session
states to 'open' (and so they skip 'opening_control'). With cash control
enabled however this does not happen.
8a232e7f45 removed a button which allowed
you to open the session. This adds back a simplified button which
appears when necessary and just sets the state to 'open'.

addons/point_of_sale/point_of_sale_view.xml

@@ -772,6 +772,9 @@
             <field name="arch" type="xml">
                 <form string="Point of Sale Session">
                     <header>
+                        <button name="open" type="workflow" string="Open Session"
+                                attrs="{'invisible' : ['|', ('cash_control', '=', False), ('state', '!=', 'opening_control')]}"
+                                class="oe_highlight"/>
                         <button name="open_frontend_cb" type="object" string="Continue Selling" states="opened"
                             class="oe_highlight"/>
                         <button name="cashbox_control" type="workflow" string="End of Session"