Draggable: Only blur the focused element if the event occurs on a handle

I manually verified this works and doesn't regress #4261 in IE 8, 9, 10, 11, and Chrome. I'm having a hell of a time writing a test for this though. The existing test for #4261 passes in all the IEs.

cc @mikesherov
I split them up to make it clear that the Support comment only applied to that check. But if I combine the comments on lines 133 and 134 I think it should be clear enough.