gui/event-editor: Add time zone selection

Creates a chooser managing a GDateTime. The chooser allows the user
to change the date, the time and the timezone independently.

This replaces the precedent GcalTimeChooserRow widget, because it is
easier to manage the different interactions with the user in a single
widget.

Co-authored-by: Georges Basile Stavracas Neto
<georges.stavracas@gmail.com>

src/utils/gcal-date-time-utils.c

@@ -299,3 +299,48 @@ gcal_date_time_add_floating_minutes (GDateTime *initial,
   offset = offset / 1000000;
   return g_date_time_add_seconds (result, offset);
 }
+
+/**
+ * gcal_date_time_add_floating_minutes:
+ * @date_time: #GDateTime for which to calculate the UTC difference
+ *
+ * Creates a string "UTC+-n" where n is the offset to UTC of date_time.
+ *
+ * Returns: (transfer full): A new string.
+ */
+gchar*
+gcal_date_time_format_utc_offset (GDateTime *date_time)
+{
+  g_autoptr (GTimeZone) local_time_zone = NULL;
+  g_autoptr (GString) utc_str = NULL;
+  gint64 utc_offset_seconds;
+
+  utc_str = g_string_new ("UTC");
+  utc_offset_seconds = g_date_time_get_utc_offset (date_time) / (1000 * 1000);
+
+  if (utc_offset_seconds != 0)
+    {
+      gboolean negative;
+      gint64 minutes;
+      gint64 hours;
+
+      negative = utc_offset_seconds < 0;
+
+      if (negative)
+        utc_offset_seconds = -utc_offset_seconds;
+
+      hours = utc_offset_seconds / 3600;
+      minutes = (utc_offset_seconds - hours*3600) / 60;
+
+      g_string_append (utc_str, negative ? "-" : "+");
+      g_string_append_printf (utc_str, "%ld", hours);
+
+      if (minutes != 0)
+        {
+          g_string_append (utc_str, ":");
+          g_string_append_printf (utc_str, "%02ld", minutes);
+        }
+    }
+
+  return g_string_free_and_steal (g_steal_pointer (&utc_str));
+}

src/utils/gcal-date-time-utils.h

@@ -57,4 +57,7 @@ ICalTimezone*        gcal_timezone_to_icaltimezone               (GTimeZone
 GDateTime*           gcal_date_time_add_floating_minutes         (GDateTime          *initial,
                                                                   gint                minutes);
 
+gchar*               gcal_date_time_format_utc_offset            (GDateTime          *date_time);
+
 G_END_DECLS
+