[libical] icaltime_compare_date_only semantics

[Dan Mosedale]

Right now, it appears to me that icaltime_compare_date_only() can't 
possibly do the right thing in all circumstances, because it always 
normalizes to UTC and then compares.  If I have two icaltime's that are 
in the same day in some timezone, but that difference spans midnight 
when normalized to UTC, the wrong thing is returned.

I propose to fix this by giving icaltime_compare_date_only() a third 
argument: timezone to do the compare in.  Both dates would be normalized 
to that timezone before comparing. What do folks think -- does this 
sound like a reasonable API change?

Dan