U2 Element: Fix replaceChild()
U2 Element: Fix replaceChild()

unload() is now properly called on the whole original Element.
load() is now called only once on the new element (previously, onReplaceChild
and replaceChild() both did it).

foam-framework/foam,jlhughes/foam,foam-framework/foam,foam-framework/foam,jlhughes/foam,foam-framework/foam,foam-framework/foam,jlhughes/foam,jlhughes/foam