ContentPane: better detection about which widget to resize (in layout mode)

[bill]

This ticket is based on #5169. If a ContentPane has a single child (possibly loaded via an href), it can act like a proxy, and reflect size changes of itself to that single child. However, doesn't work so well if there are extraneous children, such as having a LayoutContainer and a dojo.data.store child too.

[12160] lets the developer specify when ContentPane? should act as a layout container, but that still doesn't solve the problem here. As per the TODO in that checkin it should be more intelligent about finding the widget to resize, probably by looking for a widget with a resize() method.

See also related bug #5638 (which I might close as a duplicate of this one).