LibBalsaMessage structure

A recent mailing list thread raised a question about the LibBalsaMessage structure, namely why the msgno member is declared glong, when guint seems more natural. The value is in fact always cast to guint, mostly silently, when it is used.

The structure also has two gboolean members, while a third member that also represents a Boolean state is a single-bit unsigned. The structure could be compressed a little by making all three members single-bit fields.

A branch has been added to explore changes to the structure.