The following patch for the BALSA_2 branch changes the BalsaIndex message
list from a GtkCTree to a GtkTreeView. It also makes BalsaIndex a subclass
of the tree widget itself instead of the scrolled window that contains it,
which requires a few extra changes.

The code is noticeably slower than the GtkCTree version. The absence of a
`move subtree' method makes threading especially slow. Any suggestions
about speeding it up are welcome!

Files:

libbalsa/mailbox.c:
 - new MESSAGES_DELETE_ALL signal, for faster closing.
src/balsa-app.c, src/main-window.c:
 - BalsaIndex is now the child widget of a notebook page.
src/balsa-icons.[ch], src/main.c:
 - complete the move from BalsaIcon to GtkIconFactory.
src/balsa-index-threading.c, src/balsa-index.[ch], src/balsa-mblist.c:
 - replace GtkCtree with GtkTreeView and friends.
src/message-window.c, src/print.c, src/sendmsg-window.c, src/store-address.c:
 - use new API to get selected messages.