Introduce GcalEventPopover

GcalEventPopover gives a preview of the event, and handy access
to the event description and information. It also allows easily
joining meetings if any one if available.

The list of supported meeting URLs is small now, but I hope
more people will contribute as we go.

It is also possible to edit the event through an edit button.

src/gui/gcal-event-widget.h

@@ -26,10 +26,19 @@
 
 G_BEGIN_DECLS
 
-#define GCAL_TYPE_EVENT_WIDGET                    (gcal_event_widget_get_type ())
+typedef enum
+{
+  GCAL_EVENT_PREVIEW_ACTION_NONE,
+  GCAL_EVENT_PREVIEW_ACTION_EDIT,
+} GcalEventPreviewAction;
 
+#define GCAL_TYPE_EVENT_WIDGET                    (gcal_event_widget_get_type ())
 G_DECLARE_FINAL_TYPE (GcalEventWidget, gcal_event_widget, GCAL, EVENT_WIDGET, GtkBin)
 
+typedef void         (*GcalEventPreviewCallback)                 (GcalEventWidget        *event_widget,
+                                                                  GcalEventPreviewAction  action,
+                                                                  gpointer                user_data);
+
 GtkWidget*           gcal_event_widget_new                       (GcalContext        *context,
                                                                   GcalEvent          *event);
 
@@ -48,6 +57,10 @@ void                 gcal_event_widget_set_date_end              (GcalEventWidge
 void                 gcal_event_widget_set_read_only             (GcalEventWidget    *event,
                                                                   gboolean            read_only);
 
+void                 gcal_event_widget_show_preview              (GcalEventWidget          *self,
+                                                                  GcalEventPreviewCallback  callback,
+                                                                  gpointer                  user_data);
+
 /* Utilities */
 
 GtkWidget*           gcal_event_widget_clone                     (GcalEventWidget    *widget);