Convert HashMap<Key, OwnPtr<T>> to HashMap<Key, NonnullOwnPtr<T>>.

In every case I found, we never wanted to support null entry values.
With NonnullOwnPtr, we can encode that at the type level. :^)

Applications/Taskbar/WindowList.h

@@ -78,5 +78,5 @@ public:
     Function<GButton*(const WindowIdentifier&)> aid_create_button;
 
 private:
-    HashMap<WindowIdentifier, OwnPtr<Window>> m_windows;
+    HashMap<WindowIdentifier, NonnullOwnPtr<Window>> m_windows;
 };