calendar: avoid double event popup

Some browsers (e.g. chrome) trigger onEmptyClick as well as onBeforeLightbox during drag&drop which calls two slow_create calls (and two popups). Workaround to kill the second one.

Demo: Run OpenERP with Chrome 33... or 34... and then just go to Calendar view say for example Human Resources -> Leaves -> Leave Requests and then in calendar view, create multi day leave by dragging mouse on two days in month mode.

Port to github of https://code.launchpad.net/~openerp-dev/openerp-web/7.0-opw-606297-msh/+merge/218810