History: GroupCalendarDateIn17

Where I work, we still run 1.7.8, for various reasons not of general interest.

Yesterday I was looking at our calendar (group calendar, not individual one), and found that it listed Monday as the 21st of September, instead of the 20th!

The problem seems to be with incorrect attempts to reason about week numbers and dates relative to week number, in tiki-calendar.php. I backported some code from 1.8.4 that seems to do a better job. I'm attaching a patch file, in case that might help others.