Userland: Dynamically update the MonitorSettingsWidget countdown timer

This causes the number of seconds in "Do you want to keep the new
settings? They will be reverted after 10 seconds." to be dynamically
updated.

Userland/Libraries/LibGUI/MessageBox.cpp

@@ -55,6 +55,12 @@ int MessageBox::ask_about_unsaved_changes(Window* parent_window, StringView path
     return box->exec();
 }
 
+void MessageBox::set_text(String text)
+{
+    m_text = move(text);
+    build();
+}
+
 MessageBox::MessageBox(Window* parent_window, StringView text, StringView title, Type type, InputType input_type)
     : Dialog(parent_window)
     , m_text(text)