Mail notification tweaks

This merge request contains three improvements to email notifications, in separate commits. I could break them into separate merge requests, but one of them depends on the changes made in another, so this seems like less hassle.

The three improvements are:

• Always show the From and Subject headers of new messages when they arrive, even if there have been multiple notifications sent previously. This was requested in comments 16 and 18 of Bugzilla bug 653699. (As noted in the commit message, this works only if the newest message arrives by itself, so perhaps ‘always’ is a slight overstatement.)

• Hide notification when other clients read mail. AFAIK, this hasn't been previously requested, but in this modern era of email checking on smartphones and tablets, I find it to be a big improvement when Evolution doesn't leave me with a lingering nag about mail I've already read on another device.

• Integrate with libunity, to support showing the number of unread inbox messages in the Ubuntu dock (or in any GNOME Shell environment with the Dash to Dock extension). This is #9 (closed) (waves at @Filbuntu). On my desktop, it looks like this:

Closes #9 (closed).