[IMP] point_of_sale: rename Fiscal Position frontend button to Tax

Fiscal position might be a good name in the backend, but it's confusing
for POS users.

With permission from @mart-e.

addons/point_of_sale/static/src/js/screens.js

@@ -1919,7 +1919,7 @@ var set_fiscal_position_button = ActionButtonWidget.extend({
             };
         });
         self.gui.show_popup('selection',{
-            title: _t('Select Fiscal Position'),
+            title: _t('Select tax'),
             list: selection_list,
             confirm: function (fiscal_position) {
                 var order = self.pos.get_order();

addons/point_of_sale/static/src/xml/pos.xml

@@ -102,7 +102,7 @@
 
     <t t-name="SetFiscalPositionButton">
         <div class='control-button'>
-            <i class='fa fa-book' /> Fiscal position
+            <i class='fa fa-book' /> Tax
         </div>
     </t>