Fixing event.clientX to event.pageX

Corresponds to correct remark of Petroianu Daniel one should use [event.pageX](http://api.jquery.com/event.pageX/) instead of `event.clientX`. The current version of `event.clientX` works incorrectly in case of the grid having many columns and horizontal scroll position which is somewhere in the middle of the grid. The usage of `event.pageX` instead fixes the problem.