"Index" button

Hello Julien,

I'm using your application for a while and recently I figured out that
it would be very nice to be able to jump back right to the index of
man pages.

So I've prepared this little patch that adds the "Index" button to a
menu in the man page browser. Also I made a custom icon for the 
button which is based on application's icon.

Hope the patch will be useful.

Thanks.

-- Artyom