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/ToolBar.cpp

@@ -64,6 +64,7 @@ void ToolBar::add_action(Action& action)
         button.set_exclusive(true);
     button.set_action(*item->action);
     button.set_tooltip(item->action->text());
+    button.set_focus_policy(FocusPolicy::TabFocus);
     if (item->action->icon())
         button.set_icon(item->action->icon());
     else