Fix download of attachments that are part of TNEF message (#1490091)

Rcube_message_part::body content should never be modified by code out of the rcube_message.
Added convenient rcube_message::get_part_body() method, making rcube_message::get_part_content() deprecated.
Update changelog
Fix so attachment charset is set in headers of forward/draft message (#1490109)
Fix handling of uuencoded messages if messages_cache is enabled (#1490108)
Fix handling of base64-encoded attachments with extra spaces (#1490111)
Fix handling of UNKNOWN-CTE response, try do decode content client-side (#1490046)
Really fix UNKNOWN-CTE handling
Fix of really UNKNOWN-CTE handling