[MlMt] Messages not marked read on open

[John Cooper]

My preferences are set to mark messages read after 20 seconds. In every 
other mail program I've used, this setting applies only to the preview 
pane (Message View). However, when message view is off, and I 
double-click to open a message for viewing in a new window, the message 
is not marked read for 20 seconds. I can actually open a short email in 
a new window, read it, close it, and still have to mark it read 
manually. I'd prefer to have messages marked read as soon as I 
explicitly open them.