Allow infusion of widgets in other namespaces
...without hacks.
On the PHP side, custom widgets need to override
Element::getJavaScriptClassName(). The infused data will now contain the
full class name, rather than assuming a "OO.ui" prefix.
Bug: T104989
Change-Id: I7f0e74fcd9a5336ab4e4323cfe30f0ec9db342a9