event: Save local time correctly

The old code here was not setting tzid correctly. It me took hours to
figure out how to do this, but it turned out to be simple once I found
the right place, buried deep in the e-d-s source code.

Now, ideally we would not change the timezone of the event every time we
edit it. But some big changes in how GNOME Calendar tracks timezone are
going to be required if we do that. In particular, I believe we would
have to stop saving all day events in UTC, and give up on using
GDateTime as we do now, and track timezone completely separately
everywhere using icaltimezone. This is what e-d-s does, and it's
probably what we should be doing too. But that would be a big change.
This commit solves the basic problem, which is to ensure the event is
scheduled for the right time, regardless of timezone.

This finally completely fixes #168.

src/gcal-event.c

@@ -240,12 +240,8 @@ build_component_from_datetime (GcalEvent *self,
     }
   else
     {
-      gchar *tzid;
-
-      tzid = g_date_time_format (dt, "%Z");
-      comp_dt->value->zone = icaltimezone_get_builtin_timezone_from_tzid (tzid);
-      /* This transfers ownership of tzid. */
-      comp_dt->tzid = tzid;
+      comp_dt->value->zone = e_cal_util_get_system_timezone ();
+      comp_dt->tzid = g_strdup (icaltimezone_get_tzid ((icaltimezone *) comp_dt->value->zone));
     }
 
   return comp_dt;