Put a TODO for "password salts"

The way Danbooru handles passwords is simplistic to stay the least. It's
also impossible to know the "password salt" without visiting the API
page. As such, "salts" need to be put along with the board they
originate from.

The code doesn't handle missing keys, hence this TODO.

src/danbooruconnectwidget.cpp

@@ -130,6 +130,7 @@ void DanbooruConnectWidget::getWalletData()
 
         QString hashedPassword;
 
+        // TODO: Handle the case where the "salt" is not known
         hashedPassword = boardSalts.value(key);
 
         hashedPassword = hashedPassword.arg(valueMap[QLatin1String("password")]);