[FIX] web: don't scroll when navigating in editable list

Problem occured with checkboxes in editable lists: when pressing
up, down, right or left with the focus on a checkbox, in addition
to navigating in the requested direction in the list, the page
scrolled (if there was a scrollbar of course). This was because the
event wasn't 'preventDefaulted'.

addons/web/static/src/js/fields/abstract_field.js

@@ -412,18 +412,22 @@ var AbstractField = Widget.extend({
                 this.trigger_up('navigation_move', {direction: 'cancel'});
                 break;
             case $.ui.keyCode.UP:
+                ev.preventDefault();
                 ev.stopPropagation();
                 this.trigger_up('navigation_move', {direction: 'up'});
                 break;
             case $.ui.keyCode.RIGHT:
+                ev.preventDefault();
                 ev.stopPropagation();
                 this.trigger_up('navigation_move', {direction: 'right'});
                 break;
             case $.ui.keyCode.DOWN:
+                ev.preventDefault();
                 ev.stopPropagation();
                 this.trigger_up('navigation_move', {direction: 'down'});
                 break;
             case $.ui.keyCode.LEFT:
+                ev.preventDefault();
                 ev.stopPropagation();
                 this.trigger_up('navigation_move', {direction: 'left'});
                 break;