[FIX] web: fix RTL pivot view dropdown

Reproduce the issue

    - Load Arabic language
    - Install Sales
    - Report > Sales
    - Switch to arabic
    - Click on the pivot view to display the dropdown

    The dropdown position is incorrect

Cause

    The calculations are based on a `left` value who is weird.

This commit use the mouse X position to compute the `left` value.

OPW-2146210

addons/web/static/src/js/views/pivot/pivot_controller.js

@@ -291,7 +291,7 @@ var PivotController = AbstractController.extend({
         } else {
             var position = $target.position();
             var top = position.top + $target.height();
-            var left = position.left + ev.offsetX;
+            var left = ev.clientX;
             this._renderGroupBySelection(top, left);
         }
     },