Fix global widget retrieval when layout category is not "jsf"

The LayoutStore component allows to register layout definitions, widget definitions, layout types and widget types in other categories than the default one, "jsf".
This is useful when using layouts/widgets in another context than the default JSF application, to avoid name clashes, for instance.
This is useful for action widget types, as well as widget types configurations (that can contain layout definitions to represent accepted properties).

The actual problem: when a layout definition holds references to a widget, the widget can reference its own category, and when the category is not specfied, the JSF layout service assumes it's the default category, "jsf".

But it should actually be useful to use the same category than the layout (and still assume this category is "jsf" by default, as definitions are not aware of their category – actually categories can be multiple) and handling this more generically would require changing the JSF layout service API to specify the layout category.