interaction: set body and transferStatus of DATA_TRANSFER messages

This patch adds code in the interaction::Info::init function so that the
"body" and "transferStatus" fields are always set when an Info struct is
constructed for a message of type DATA_TRANSFER.

This removes some code duplication in conversationmodel.cpp, where these
fields were being set as an extra step after construction in three
different places.

It also fixes a bug in the ConversationModelPimpl::slotMessageUpdated
function, which did *not* set the "body" of DATA_TRANSFER messages. The
body was therefore empty instead of containing a file path, which is
what caused the image preview bug described in the following issue:
GitLab: #1671

The patch also reverts a change that was made in the
MessageListModel::update function by commit
d2eba1d9. This change was a workaround
for the above bug, but it is no longer necessary (and it broke message
deletion, which relies on the body of the deleted message being set to
the empty string).
GitLab: #1825

Change-Id: I5848b93a12c1ef7b3735c5c6db6b32a9bbc4041d