Replace remaining 0 with nullptr in Qt code

Also used type-appropriate enum values such as Qt::NoItemFlags in
some cases.

All cases identified via -Wzero-as-null-pointer-constant

src/qt/networkstyle.cpp

@@ -88,5 +88,5 @@ const NetworkStyle *NetworkStyle::instantiate(const QString &networkId)
                     network_styles[x].titleAddText);
         }
     }
-    return 0;
+    return nullptr;
 }