Introduce transaction setter xaccTransSetDatePostedSecsNormalized() that ignores the time-of-day part.

We've struggled with the time-of-day part of the PostedDate for long
enough. The PostedDate field is just not meaningful with anything else
but a plain date, and no time-of-day at all. Hence, the correct setter
function for this particular field must ignore the time-of-day. Consequently,
a GDate should be used here anyway, but in many places the time64 is more
convenient. The new function will now redirect that time64 to the GDate
setter function to make sure we will now map everything to one single date.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23044 57a11ea4-9604-0410-9ed3-97b8803252fd