Add QML support for QMailAccountListModel and QMailMessageListModel classes

The setRoleNames function is not used in the constructors of classes QMailAccountListModel and QMailMessageListModel. Hence, the associated roles cannot be easily accessed from QML code. I suggest that the constructors of these classes would register the associated roles with the setRoleNames function so that the role data could be easily accessed from a QML delegate. For example, by adding the following code to the constructor of class QMailAccountListModel

QHash<int,QByteArray> roles;
roles[NameTextRole] = "name";
setRoleNames(roles);

the name of an account can be queried in a QML delegate as follows

Component {
id: accountListDelegate
Text

{ text: name }

}