[MlMt] Ticket opened: Active area in new message windows

[John Cooper]

I opened a ticket about a minor issue that's tripped me up several 
times.

When a message opens in its own window, the message header pane is 
active instead of the message body pane, so pressing the space bar to 
read further only expands (or closes) the message header. This is a less 
frequent need for most users and a less intuitive use of the space bar. 
Unless the reader notices the header pane change, it seems that the 
space bar has no effect.

When a message opens, make the message body bane active by default.

#928