LibGUI: Add Widget focus policies

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.

Libraries/LibWeb/OutOfProcessWebView.cpp

@@ -40,6 +40,7 @@ namespace Web {
 OutOfProcessWebView::OutOfProcessWebView()
 {
     set_should_hide_unnecessary_scrollbars(true);
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
     m_client = WebContentClient::construct(*this);
     client().post_message(Messages::WebContentServer::UpdateSystemTheme(Gfx::current_system_theme_buffer_id()));
 }