lib: Add new, public notmuch_message_get_filenames
This augments the existing notmuch_message_get_filename by allowing
the caller access to all filenames in the case of multiple files for a
single message.
To support this, we split the iterator (notmuch_filenames_t) away from
the list storage (notmuch_filename_list_t) where previously these were
a single object (notmuch_filenames_t). Then, whenever the user asks
for a file or filename, the message object lazily creates a complete
notmuch_filename_list_t and then:
For notmuch_message_get_filename, returns the first filename
in the list.
For notmuch_message_get_filenames, creates and returns a new
iterator for the filename list.