This isn't entirely symmetrical with on_load_start as it will also fire
on reloads and back/forward navigations. However, it's good enough for
some basic use cases, and we can do more sophisticated notifications
later on when we need them.
Every widget now has a GUI::FocusPolicy that determines how it can
receive focus:
- NoFocus: The widget is not focusable (default)
- TabFocus: The widget can be focused using the tab key.
- ClickFocus: The widget can be focused by clicking on it.
- StrongFocus: Both of the above.
For widgets that have a focus proxy, getting/setting the focus policy
will affect the proxy instead.
