Fix `zF` double clicking on browser elements (#757)
`nsIDOMWindowUtils.dispatchDOMEventViaPresShell()` dispatches
a 'command' event automatically after a 'click' event on XUL pages.
This was causing `zF` to effectively double click on browser elements,
since VimFx also simulates a 'command' event.
Elements which suffers from this double click include but are not
limited to, the new tab button, question mark circles in the preferences
page and buttons created by the add-on sdk.