When transferring multiple files via Bluetooth, we quickly receive
one transfer request after another, so this causes a timing issue
where the engine may quit immediately after the first request and
is in the process of closing down when the next request is received,
and thus cannot be started again for the next request.

Review fixes

MNotificationGroup::publish() sets its x-nemo-preview-body field
using the preview body of the last added notification group. This
means if the group is published before the notification, the
preview body used will be that of the last published notification
group instead of the current one.

If the group is published after the notification instead, the
notification will ensure the group properties are set correctly
before the group is published.

Allow MediaItem::ResourceName to be passed to sendNotification() when
MediaItem::Url is empty, as it will be if an uploader is sending a
non-file resource such as a vCard.