Make cofigurable the no threads message in the Inbox block

Problem/Motivation

A common usage of the Private message module is to show the Inbox side by side with the list of messages.
With this display, when there are no threads, the user will see "You do not have any private messages" in the Inbox block and "You do not have any messages" in the messages list. Both in a very short area. This looks weird.

At the moment there is no configuration to change these messages.

Proposed resolution

My proposal is to make the no threads message in the block part of the configuration of the block. That way, an user can make this empty (or whatever he chooses).

Remaining tasks

Decide what it the better option and patch.