Partially fixes NB#77528

Solves a memory problem when obtaining the size of an attachemnt
   * modest-count-stream.[ch] : implementation of TnyStream to get the size of a stream withou wasting extra memory
   * widgets/modest-attachment-view.c : uses the former to get the size of attachments
   * modest-mail-operation.c : removed two redundant lines
   * modest-tny-send-queue.c : check for NULL value before a strdup()

pmo-trunk-r3870