Update ChildViewLayoutDemo in prep for FlexGrid layout.

This branch is **NOT** ready to be merged yet, but I wanted to go ahead and get some feedback on how to move forward to get this ready for a FlexGrid layout demo.

I've updated the demo to include a TabView with both Vertical and Horizontal views so that you can see how each work and so that we can add the FlexGrid later.

At the moment, the resizing is working (although the CSS cursor and image need to be updated to match the horizontal style) but the dragging is not working. It appears that the drag code is specific to vertical and I'm afraid I'm not familiar enough with how it works to update it.

@publickeating Can you perhaps update the `dragUpdated` function to account for horizontal drags?

@nicolasbadia You want to go ahead and add another tab item for your flex grid layout?