#164 onmouseover not recommended for OpenerWithImageCell

[Joshua Hewitt]

OpenerWithImageCell should only be used with onclick as it does not work very well with mouseover/out. For it to work properly it should implement a onBrowserEvent similar to DelayEventCell's - need to fire listeners.onMouseLeave rather than listeners.fireMouseEvent. Added a TODO comment in the class javadoc.

Not urgent, just something we need to take into account when joining DOM & remote tables.