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/InProcessWebView.cpp

@@ -68,6 +68,7 @@ InProcessWebView::InProcessWebView()
 {
     set_should_hide_unnecessary_scrollbars(true);
     set_background_role(ColorRole::Base);
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
 
     m_copy_action = GUI::CommonActions::make_copy_action([this](auto&) {
         GUI::Clipboard::the().set_plain_text(selected_text());