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/TextBox.h

@@ -33,7 +33,7 @@ namespace GUI {
 class TextBox : public TextEditor {
     C_OBJECT(TextBox)
 public:
-    explicit TextBox(Widget* parent);
+    explicit TextBox(Widget* parent = nullptr);
     virtual ~TextBox() override;
 };