Support for large media transmission.

1) All attachments are now treated as streams.  A full
   attachment is never buffered in memory (except for scaled MMS
   PDU creation in that transport).

2) Fix for some of the encrypting streams.

3) Cache thumbnails in the part database. Otherwise thumbnail
   generation takes too long for larger media.

4) Eliminate part "provider."

Remaining work:

1) Provide internal mechanism for viewing attachments.

2) Provide in-conversation item progress bars for attachment
   upload and download.

3) Provide options for limiting attachment download over mobile
   data.
add aspect ratio to part metadata

// FREEBIE
fix content parsing crash