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/LibVT/TerminalWidget.h

@@ -85,8 +85,6 @@ public:
     void paste();
     void clear_including_history();
 
-    virtual bool accepts_focus() const override { return true; }
-
     Function<void(const StringView&)> on_title_change;
     Function<void()> on_command_exit;