What is the problem or limitation you are having?

Similar to #2307

Right now, unlike with URL, to create a WebView and set [HTML] content to it, we have to do it in two steps:

web_view = toga.WebView()

web_view.set_content("", html_content)

Describe the solution you'd like

WebView already allows passing a URL during instantiation, so why not allow passing "content" as well? That way we can do it in one step, and be consistent with MainWindow and ScrollContainer:

web_view = toga.WebView(content=html_content)

Describe alternatives you've considered

None

Additional context

No response