squashed single 4-0 backport commit to sync 4-0 mailmerge.py
with that of current master to get a mailmerge.py that works
with python3 and python2, and successfully

a) sends utf-8 plain text
b) html
c) odt, pdf, doc attachments

(cherry picked from commit c25bee9c)

i118736 - i118787 : fix XMailMessage implementation in mailmerge.py

(cherry picked from commit 4166969f)

(cherry picked from commit 3f2f92b7)

i118791 - Encode ReadableName in UTF-8 only when necessary

(cherry picked from commit a5cefd40)

i118814 - Allow set connection timeout in Mail API

(cherry picked from commit 8a324a3b)

make emailmerge work for me with python3

(cherry picked from commit e48a060e)

Change-Id: I6289b522513a2fc86e261c85a04ca9c89fd55b63
Reviewed-on: https://gerrit.libreoffice.org/1713Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>