lib-imap-storage: Added support for getting BODYPARTSTRUCTURE Based on patch by Stephan Bosch.

lib-imap-storage: Changed imap_msgpart_url API. Adds enum mail_error return value to imap_msgpart_url_open_mailbox(). Now parses msgpart at the beginning and adds function to access underlying imap_msgpart object directly.

Error handling API changes to previous IMAP URL related changes.

Created lib-imap-storage for IMAP-specific functionality that uses lib-storage. This is done to prevent introducing dependencies on lib-storage in lib-imap. It contains IMAP message part access functionality, including URL-based access.