xilem_core: Add a WithoutElements ViewSequence

[Philipp-M]

This is similar as the fork view, but it allows using Views/ViewSequences without elements within a ViewSequence context that contains elements, see #461 (comment) for the original motivation.

Taking xilem_web as example, this should be possible (as well as something similar as in the linked comment):

fn run_once_fragment() -> impl DomFragment<()> {
    without_elements(run_once(|| {}))
}

The WidgetViewSequence is "fixed" as well, by using the "right" widget type (DynWidget is only used in AnyView, whereas I think Box<dyn Widget> is more general and correct).