event-editor: Improve event name validation handling

- Fix a bug where clicking the "Add Event" button created an empty title in the event editor dialog.
- Add "error" class when the user writes an invalid event name.
- Add new function to validate event name.

Because the sections are split into files, the event editor will need a signal to check that each section is correct, and each section will have to emit the signal. This will need to be done in a future commit.

src/utils/gcal-utils.c

@@ -1343,3 +1343,18 @@ gcal_util_get_app_timezone_or_local ()
   context = gcal_application_get_context (application);
   return g_time_zone_ref (gcal_context_get_timezone (context));
 }
+
+/**
+ * gcal_is_valid_event_name:
+ * @event_name: the name of the event to check
+ *
+ * Check if the event name is valid
+ *
+ * Returns: %TRUE if the name is valid, %FALSE otherwise.
+ */
+gboolean
+gcal_is_valid_event_name (const gchar *event_name)
+{
+  g_autofree gchar *aux = g_strstrip (g_strdup (event_name));
+  return g_utf8_strlen (aux, -1) > 0;
+}

src/utils/gcal-utils.h

@@ -138,4 +138,6 @@ const gchar *        gcal_util_translate_time_string             (const gchar
 
 GTimeZone *          gcal_util_get_app_timezone_or_local         ();
 
+gboolean             gcal_is_valid_event_name                    (const gchar          *event_name);
+
 #endif /* __GCAL_UTILS_H__ */