[MlMt] Centering mainOutline view

[Mike Petonic]

Hi Benny --

Any chance to get a selector to work in the mainOutline to work like the 
default ^l selector in OS X (and Emacs):

```
	"^l"        = "centerSelectionInVisibleArea:";  /* C-l          
Recenter */
```

Sometimes when you're scrolling or going to certain messages in a 
thread, the cursor is damn close to the end or is even past the viewable 
scroll area.  A simple ^l would normalize the view.

Thanks!  I'm still in "new car smell" (new customer) mode of MailMate.  
It totally rocks!
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