LibGUI: Don't require passing a parent to widget constructors

This is a step towards using Core::Object::add<T> more, which takes
care of parenting the newly created child automatically.

Libraries/LibGUI/ListView.h

@@ -33,7 +33,7 @@ namespace GUI {
 class ListView : public AbstractView {
     C_OBJECT(ListView)
 public:
-    explicit ListView(Widget* parent);
+    explicit ListView(Widget* parent = nullptr);
     virtual ~ListView() override;
 
     int item_height() const { return 16; }