Added event param to getHorizontalBounds

Firefox (v30) errors on getHorizontalBounds() because **event** is out of scope. This occurs when **allowImages** is enabled, and causes the Insert Image tool tip to not work. 

Request to add **event** to the parameters of getHorizontalBounds()!

Btw, can someone explain why in chrome **event** is *not* undefined at 
*coordY = event.pageY - root.scrollY;* 
(line 206 in getHorizontalBounds())? Is it just being generous with the scope, or is it supposed to work like that?