LibGUI: More work on GInputBox.

- If the GInputBox has a parent and the parent is a GWindow, center the
  input box window within the parent window. This looks quite nice.

- Stop processing events in a nested event loop immediately after it's
  been asked to quit.

- Fix GWidget::parent_widget() behavior for non-widget parents.

LibGUI/GInputBox.h

@@ -2,10 +2,11 @@
 
 #include <LibGUI/GDialog.h>
 
+class GButton;
+class GTextEditor;
+
 class GInputBox : public GDialog {
 public:
-    enum ExecResult { ExecOK = 0, ExecCancel = 1 };
-
     explicit GInputBox(const String& prompt, const String& title, GObject* parent = nullptr);
     virtual ~GInputBox() override;
 
@@ -15,4 +16,8 @@ private:
     void build();
     String m_prompt;
     String m_text_value;
+
+    GButton* m_ok_button { nullptr };
+    GButton* m_cancel_button { nullptr };
+    GTextEditor* m_text_editor { nullptr };
 };