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.

Applications/HexEditor/HexEditor.cpp

@@ -42,6 +42,7 @@
 
 HexEditor::HexEditor()
 {
+    set_focus_policy(GUI::FocusPolicy::StrongFocus);
     set_scrollbars_enabled(true);
     set_font(Gfx::Font::default_fixed_width_font());
     set_background_role(ColorRole::Base);

Applications/HexEditor/HexEditor.h

@@ -74,7 +74,6 @@ protected:
     virtual void mouseup_event(GUI::MouseEvent&) override;
     virtual void mousemove_event(GUI::MouseEvent&) override;
     virtual void keydown_event(GUI::KeyEvent&) override;
-    virtual bool accepts_focus() const override { return true; }
 
 private:
     bool m_readonly { false };