[patch] two changes for balsa 1.2.0

[Willem Riede <wriede riede org>]

I have recently started using Balsa, and I like it a lot, but
since I am in the habit of weeding out uninteresting messages
from some of the high volume mailing lists I subscribe to by
deactivating the preview pane and then selecting ranges of
messages to delete, and then re-activating the preview pane 
and moving from unread message left to unread message and read 
them in the preview pane, I had two issues with Balsa:

1) it didn't have a one-click way to dis-/enable the preview pane;
2) with the preview pane off, messages following a range to delete
   did get their 'new' flag taken away, just because they got
   selected as a side effect, not because the content got displayed.

Since you'll likely say 'show me the code', I've made the changes
myself and offer the attached patch for anyone to try and if the
community likes it, be my guest to incorporate it.

By the way, I'm not much of an artist, so if anyone wants to 
suggest a better pixmap for the toolbar icon I made to toggle the
preview pane, I won't be offended.

The status of the patch is 'it works for me', but it might have 
undesirable side effects that I've not discovered for all I know...

Regards, WIllem Riede.

wr-balsa.diff