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/LibGUI/ComboBox.cpp

@@ -85,7 +85,7 @@ ComboBox::ComboBox()
     };
 
     m_open_button = add<ControlBoxButton>(ControlBoxButton::DownArrow);
-    m_open_button->set_focusable(false);
+    m_open_button->set_focus_policy(GUI::FocusPolicy::NoFocus);
     m_open_button->on_click = [this](auto) {
         if (m_list_window->is_visible())
             close();