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/Dialog.cpp

@@ -31,8 +31,8 @@
 
 namespace GUI {
 
-Dialog::Dialog(Core::Object* parent)
-    : Window(parent)
+Dialog::Dialog(Window* parent_window)
+    : Window(parent_window)
 {
     set_modal(true);
 }