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/Menubar.h

@@ -32,7 +32,7 @@ public:
 private:
     Menubar() = default;
 
-    NonnullRefPtrVector<Menu> m_menus;
+    Vector<NonnullRefPtr<Menu>> m_menus;
 };
 
 }