event: fix timezone handling

Using libical with libecal means that the dates
are always stored as UTF dates, and we receive the
timezone as the ECalComponentDateTime::tzid value.

This fact was net being considered when adding date
support on GcalEvent, and was causing various issues
when displaying dates.

Fix that by properly handling timezones in GcalEvent.

src/gcal-utils.h

@@ -59,8 +59,7 @@ icaltimetype*   datetime_to_icaltime                            (GDateTime
 
 gboolean        datetime_is_date                                (GDateTime             *dt);
 
-GDateTime*      icaltime_to_datetime                            (const icaltimetype    *date,
-                                                                 GTimeZone            **timezone);
+GDateTime*      icaltime_to_datetime                            (const icaltimetype    *date);
 
 icaltimetype*   gcal_dup_icaltime                               (const icaltimetype    *date);
 
@@ -122,4 +121,6 @@ void            get_source_parent_name_color                    (GcalManager
                                                                  gchar                **name,
                                                                  gchar                **color);
 
+gchar*          format_utc_offset                               (gint                   offset);
+
 #endif // __GCAL_UTILS_H__