onMouseUp doesn't fire to save selection if mouse is moved out of vega element

[NateLanza]

When selecting in the element view, if one drags the mouse outside the VegaLite element, the onMouseUp action that saves the selection doesn't fire. This is likely easily solved with an onMouseOut listener firing the same function.

[NateLanza]

Backlogged as I'm not currently sure how to fix this- we could track whether the mouse is in the element IF vega bubbled onMouseLeave or onMouseEnter but it doesn't appear to... not high-priority, so can be looked into later.