[mms] Determination of approximate part size should be done at IMAP level, not within Horde_Mime_Part.
approx param should be removed from Horde_Mime_Part
Better internal handling of bytes vs. size

setBytes() is only needed in cases where we don't have body data and we
don't have exact information about body size.

If 'size' disposition parameter exist, we have to assume that is the
most accurate value, so prefer that value over any manually set value
via setBytes().
Don't use approximate value from Horde_Mime_Part - this is handled by the IMAP library now