[Evolution] improvement for e-table and message list

[Chris Toshok <toshok hungry com>]

couple of changes:

I changed e-cell's to not have any knowledge of whether or not they're
editable.  the editable state always comes from the model.  Before, the
cell's flags were consulted, and the model was always ignored for this
information.

also, the message list doesn't call select_msg from the row selection
signal handler anymore - it registers an idle function which selects the
msg.  this has the primary benefit of allowing the selection to be
updated before the message loads, which is soooooo nice when you're
using imap or nntp and the message loads slowly.

the message list now updates its selection instantaneously for nntp
(mbox i presume was fast before this change), with the message area
lagging behind as the message is loaded.

Chris