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/AbstractView.h

@@ -87,7 +87,6 @@ public:
     bool is_multi_select() const { return m_multi_select; }
     void set_multi_select(bool);
 
-    virtual bool accepts_focus() const override { return true; }
     virtual void did_update_model(unsigned flags);
     virtual void did_update_selection();