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/Libraries/LibGUI/Wizards/WizardDialog.h

@@ -49,6 +49,6 @@ private:
     RefPtr<Button> m_next_button;
     RefPtr<Button> m_cancel_button;
 
-    NonnullRefPtrVector<AbstractWizardPage> m_page_stack;
+    Vector<NonnullRefPtr<AbstractWizardPage>> m_page_stack;
 };
 }