Add a checkbox to retain inbox messages.

While checked, inbox messages will not be deleted.
Background color for inbox messages near deletion.

This change causes the background color in the listUI (on the left) to
change for inbox messages are are near (gray) deletion or will be
imminently deleted (red).
Delete inbox messages while open.

With this change, inbox messages will be deleted while the client is
open, as opposed to only when the client is restarted, as was previously
the case.
Also delete outbox messages while running.

Similar to the previous patches to delete inbox messages while Pond is
running (as opposed to just when closing), this change deletes outbox
messages during normal operation.
Add command-line utility to decrypt detachments.