Fix #683: SplitView: Nested animations

SplitView's resize animation helper, transformWithTransition, needs to be
more strict when listening to onTransitionEnd. It needs verify that the
element and transition property match before completing the animation
promise.

Otherwise, the promise may complete prematurely due to animations on
descendants or animations on other properties.
Fix #683: SplitView: Position relative descendants

Currently, if you put an element with position: relative/absolute inside
of the SplitView's content, this element will end up being rendered on top
of the overlayed pane when it opens. For example, you can see this by
putting a SearchBox inside of the SplitView's content.

To avoid this, the SplitView's pane and content are each given a z-index
so that:
  1. They are each a stacking context so their descendants can't escape
     them.
  2. The pane element is always rendered on top of the content element.