Everywhere: Stop using NonnullRefPtrVector

This class had slightly confusing semantics and the added weirdness
doesn't seem worth it just so we can say "." instead of "->" when
iterating over a vector of NNRPs.

This patch replaces NonnullRefPtrVector<T> with Vector<NNRP<T>>.

Userland/Applications/GamesSettings/CardSettingsWidget.cpp

@@ -63,7 +63,7 @@ private:
 
         auto background_color = this->background_color();
         for (auto& stack : stacks())
-            stack.paint(painter, background_color);
+            stack->paint(painter, background_color);
     }
 };