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/Window.h

@@ -175,7 +175,7 @@ public:
     void apply_icon();
     const Gfx::Bitmap* icon() const { return m_icon.ptr(); }
 
-    Vector<Widget*> focusable_widgets() const;
+    Vector<Widget*> focusable_widgets(FocusSource) const;
 
     void schedule_relayout();