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/LibWeb/InProcessWebView.h

@@ -61,8 +61,6 @@ public:
 
     void set_should_show_line_box_borders(bool value) { m_should_show_line_box_borders = value; }
 
-    virtual bool accepts_focus() const override { return true; }
-
     GUI::Action& select_all_action() { return *m_select_all_action; }
     GUI::Action& copy_action() { return *m_copy_action; }