LibGUI: Don't use Core::Object::add() to instantiate dialogs

Now that add() returns a WidgetType&, we can't rely on the parent of a
GUI::Dialog to still keep it alive after exec() returns. This happens
because exec() will call remove_from_parent() on itself before
returning.

And so we go back to the old idiom for creating a GUI::Dialog centered
above a specific window. Just call GUI::Dialog::construct(), passing
the "parent" window as the last parameter.

Libraries/LibGUI/ColorPicker.h

@@ -38,7 +38,7 @@ public:
     Color color() const { return m_color; }
 
 private:
-    explicit ColorPicker(Color, Core::Object* parent = nullptr);
+    explicit ColorPicker(Color, Window* parent_window = nullptr);
 
     void build();