event-editor: Fix the "Save" / "Done" button label

The "Done" button was apparently meant for read-only events,
while the "Save" action was for everything else.
It didn't work correctly because of a mismatch between code and UI XML.

Fixes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1142

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

@@ -647,7 +647,7 @@ gcal_event_editor_dialog_class_init (GcalEventEditorDialogClass *klass)
   properties[PROP_WRITABLE] = g_param_spec_boolean ("writable",
                                                     "Whether the current event can be edited",
                                                     "Whether the current event can be edited or not",
-                                                    TRUE,
+                                                    FALSE,
                                                     G_PARAM_READWRITE | G_PARAM_EXPLICIT_NOTIFY | G_PARAM_STATIC_STRINGS);
 
   g_object_class_install_properties (object_class, N_PROPS, properties);
@@ -684,8 +684,6 @@ gcal_event_editor_dialog_init (GcalEventEditorDialog *self)
   };
   gint i = 0;
 
-  self->writable = TRUE;
-
   /* Actions */
   self->action_group = g_simple_action_group_new ();
   g_action_map_add_action_entries (G_ACTION_MAP (self->action_group),