sameValue to min-max -- choosing handle based on drag direction

[saikatguha]

Current slider behavior is setting z-index on last moved slider to enable dragging the same slider. This is useful for overlap scenarios where min-max are close to each other (but not same value) such that the handle zone overlaps.

In the special case where min-max are the same value (with allowSameValues), it creates a problem where dragging the handle only works in one direction (the direction from which the last handle came). It requires some gymnastics if the user needs to move in the opposite direction. 1) First move the sameValue handle in the other direction to see both handles again, 2) Then move the real handle in the desired direction, and finally 3) move the other handle back to the same value location. Very counter intuitive.

A better approach would be to special case the sameValue situation where moving the sameValue handle to the left moves the min handle, and moving the sameValue handle to the right moves the max handle.