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/Button.h

@@ -58,11 +58,8 @@ public:
 
     void set_action(Action&);
 
-    virtual bool accepts_focus() const override { return m_focusable; }
     virtual bool is_uncheckable() const override;
 
-    void set_focusable(bool b) { m_focusable = b; }
-
 protected:
     explicit Button(const StringView& text = {});
     virtual void paint_event(PaintEvent&) override;
@@ -72,7 +69,6 @@ private:
     Gfx::ButtonStyle m_button_style { Gfx::ButtonStyle::Normal };
     Gfx::TextAlignment m_text_alignment { Gfx::TextAlignment::Center };
     WeakPtr<Action> m_action;
-    bool m_focusable { true };
 };
 
 }