Use subject for window title of new message window

[mcclurgm]

Problem

The window title for the composer window is not very useful. It provides the bare minimum of info, but doesn't help to distinguish between (for example) multiple composer windows.

Proposal

Use the subject field to set the window title. I can think of two forms this could take: either the "New Message" title is replaced entirely by the contents of the subject field, or we could concatenate them like "New Message: Do the Thing" (where "Do the Thing" is the subject).

Prior Art (Optional)

Outlook's composer window is purely the subject. If the subject does not have any contents, the window is titled "Untitled". If there is a subject, that is the title of the window.