kimap: add methods to set/get the internal date for an APPEND query

This patch adds the methods AppendJob::setInternalDate() and
AppendJob::internalDate().
Those can be used to set the internal date for the APPEND command, see
http://tools.ietf.org/html/rfc3501#section-6.3.11 .

If setInternalDate() is not called, the exact same query is generated as
before, so existing users of AppendJob should not break.

This is needed to fix bug#230635, see
https://git.reviewboard.kde.org/r/113254/

CCBUG: 230635
REVIEW: 113253