event-editor: Focus "Done" for read-only events

In commit 1494f7407, we focus the summary entry when opening the dialog.
However, this prevents the Escape keyboard shortcut from working
when opening a read-only event after having opened a writable event.

Set focus onto the "Done" button when showing a read-only event,
as it's the only widget that can be activated/clicked in this situation.

This makes the "Escape", "Spacebar" and "Enter" keyboard shortcuts work.

Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1180

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

@@ -787,6 +787,9 @@ gcal_event_editor_dialog_set_event (GcalEventEditorDialog *self,
   self->event_is_new = new_event;
   gtk_widget_set_visible (self->delete_button, !new_event);
 
+  if (!self->writable)
+    gtk_widget_grab_focus (self->done_button);
+
 out:
   flags = GCAL_EVENT_EDITOR_FLAG_NONE;