Move resizer element higher in DOM for SplitView

This is for #9706
Replaces #9709 

This PR moves the resizer div out of the left pane and into parent DIV which puts the resizer DIV above both panes in the DOM so that it is always accessible. By design, the resizer attaches it self to one edge of the thing it's resizing but, in this case, we want it above the container.