utils: Add GDateTime utils

This will greatly help with the transition to GDateTime.
In the future, we can propose these APIs to GLib itself.

src/utils/gcal-utils.h

@@ -19,7 +19,9 @@
 #ifndef __GCAL_UTILS_H__
 #define __GCAL_UTILS_H__
 
+#include "gcal-date-time-utils.h"
 #include "gcal-manager.h"
+
 #include <gtk/gtk.h>
 #include <libecal/libecal.h>
 #include <libgweather/gweather.h>
@@ -31,7 +33,6 @@
 #define MINUTES_PER_DAY 1440
 #define MAX_MINUTES     (7 * MINUTES_PER_DAY)
 
-#define gcal_clear_datetime(dt) g_clear_pointer (dt, g_date_time_unref)
 #define gcal_clear_timeout(pp) { if (pp && *pp) { g_source_remove (*pp); *pp = 0; } }
 
 #if !EDS_CHECK_VERSION (3, 31, 90)