gcal-window: datetime settings shortcut

Shortcut is added so that time-related system settings can be easily
accessed. See #1023

The code is largely just a copy-paste of the code that opens the
"Online Accounts…" menu.

Fixes #1023

src/gui/gcal-window.c

@@ -425,6 +425,16 @@ on_window_new_event_cb (GSimpleAction *action,
   gtk_widget_set_visible (GTK_WIDGET (self->event_editor), TRUE);
 }
 
+static void
+on_window_open_date_time_settings_cb (GSimpleAction *action,
+                                      GVariant      *param,
+                                      gpointer       user_data)
+{
+  GApplication *application = g_application_get_default ();
+
+  gcal_utils_launch_date_time_settings_panel (g_application_get_dbus_connection (application), NULL, NULL);
+}
+
 static void
 on_window_open_online_accounts_cb (GSimpleAction *action,
                                    GVariant      *param,
@@ -1095,6 +1105,7 @@ gcal_window_init (GcalWindow *self)
     {"change-view", on_view_action_activated, "i" },
     {"next-date", on_window_next_date_activated_cb },
     {"new-event", on_window_new_event_cb },
+    {"open-date-time-settings", on_window_open_date_time_settings_cb },
     {"open-online-accounts", on_window_open_online_accounts_cb },
     {"previous-date", on_window_previous_date_activated_cb },
     {"show-calendars", on_show_calendars_action_activated },

src/gui/gcal-window.ui

@@ -302,6 +302,10 @@
   
   <menu id='menu'>
     <section>
+      <item>
+        <attribute name="label" translatable="yes">_Date &amp; Time Settings…</attribute>
+        <attribute name="action">win.open-date-time-settings</attribute>
+      </item>
       <item>
         <attribute name="label" translatable="yes">_Online Accounts…</attribute>
         <attribute name="action">win.open-online-accounts</attribute>