Fix roundtrip conversion of datetimes.

QDateTime.toString() is writing out milliseconds since change
15da0a5af20fe6771bcb94ef8d46edbd5c8fb64c. Unfortunately this breaks
QDateTime::fromString() with Qt::TextDate which can't handle the new
format.

Fix by making QDateTime::fromString split up seconds and milliseconds
on a period, if any. Now
QDateTime dt = ...;
assert(QDateTime::fromString(dt.toString(), Qt::TextDate) == dt)
works again.

Change-Id: Ibfe9032e357ceaf894e33f3e33affe94f56dbf5c
Reviewed-by: Lars Knoll <lars.knoll@digia.com>