Rewritten compose attachment code

Attachment now represented by the IMP_Compose_Attachment object.

Always use VFS to store attachments - if VFS not active, create a dummy
VFS object using the local temporary filesystem settings

Previously, we were using 2 separate methods to attach a message.
Consolidate attachment code in single method (_addAttachment()) so we
can do sanity checking/data analysis (e.g. MIME detection) in a single
location.