[MlMt] Multimonitor support

[Gustavo Daniel Villarreal]

I didn't want to hijack the Dark Mode thread, but I would rather see 
MailMate correctly support multiple monitors before handling a Dark 
Mode! :)

To remind Benny, MailMate doesn't remember the position of the last 
composed email if the message was composed in a different monitor than 
the main application window, or in other words, a new compose window is 
always created in the monitor where MailMaite's main window is. This did 
work before Lion I believe, where Apple addeded several multi-monitor 
options.

Thank you.