event-editor/event-editor-dialog: Store duplicate event instance

This is a preliminary step for moving the comparison and
application logic of each section into the dialog.

Essentially, the dialog will pass the duplicate event to each
section, and these sections will modify the event. If the user
presses "Save", the dialog will compare the edited event to
the original event, and apply if they are different.

This also adds the `get_event()` method which will be used
in the future.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/711>

src/gui/event-editor/gcal-event-editor-dialog.h

@@ -32,6 +32,8 @@ G_DECLARE_FINAL_TYPE (GcalEventEditorDialog, gcal_event_editor_dialog, GCAL, EVE
 
 GtkWidget*           gcal_event_editor_dialog_new                (void);
 
+GcalEvent           *gcal_event_editor_dialog_get_event          (GcalEventEditorDialog *self);
+
 void                 gcal_event_editor_dialog_present_event      (GcalEventEditorDialog *self,
                                                                   GtkWidget             *parent,
                                                                   GcalEvent             *event,