[nemo-qml-plugin-email] Add timestamp sorting to sort by sender/recipients

A helper method could be used here for gathering the duplicated code from sortByRecipient, sortBySender, sortBySubject, and sortBySize under one method:

```
void EmailMessageListModel::sortByTime(int order, EmailMessageListModel::Sort sortBy)
```
A method that would always sort in DescendingOrder.

sortByRecipiencts(int order) would change to:
```
sortByTime(order, Recipeints);
```