Added support to obtain properties of the event

Added very initial support to handle obtention of the modified properties
of the edited event. There's still some kind of Model implementation to keep
track of the changes made by the user.

src/gcal-utils.h

@@ -46,6 +46,14 @@ typedef enum
   GCAL_VIEW_MODE,
 } GcalEditableMode;
 
+typedef enum
+{
+  EVENT_SUMMARY = 0,
+  EVENT_START_DATE,
+  EVENT_LOCATION,
+  EVENT_DESCRIPTION,
+} EventEditableProperty;
+
 typedef
 const gchar*  (*GcalTranslateFunc)                              (GtkWidget             *source_widget);