Dropdown selection added for IMAP & SMTP security config

(attempt No. X)
As discussed by the main developers of the master branch, two dropdown
boxes have been added with subsequent prefilling of the port fields.

IMAP security "none","ssl","tls"
-> IMAP port  "143","993"

SMTP security "none","ssl","tls"
-> SMTP port "25","465"

There are new challenges by adding these boxes - it breaks the original
"design". But at least the functionality is given.