HorizontalLayout does not calculate drop index correctly

Found in SDK 4.0.0.13099

There's a bug in calculateDropIndex(x:Number, y:Number):int.

Line 1921:
var centerX:Number = elementBounds.x + elementBounds.height / 2;
Should be:
var centerX:Number = elementBounds.x + elementBounds.width / 2;

Looks like the code was copied from VerticalLayout and that bit was not properly updated.

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Steps to reproduce:
1. Create list with horizontal layout.
2. Make sure item renderers width is different from height (otherwise bug won't show).
3. Enable drag&drop and try reordering elements.
4. Ovserve drop indicator.

Actual Results:
Drop indicator does not skip to the other side properly when mouse cursor moves over center of list element.

Expected Results:
Drop indicator should skip to the other side as soon as mouse cursor moves over center of list element.

Workaround (if any):
Subclass HorizontalLayout and override calculateDropIndex.