quick-add-popover: focus "Create" button after selecting an agenda

So that the keyboard flow is smooth and clean.

https://bugzilla.gnome.org/show_bug.cgi?id=770714

src/gcal-quick-add-popover.c

@@ -202,6 +202,9 @@ select_row (GcalQuickAddPopover *self,
   /* Return to the events page */
   gtk_stack_set_visible_child_name (GTK_STACK (self->stack), "events");
 
+  /* Focus back the "Create" button */
+  gtk_widget_grab_focus (self->add_button);
+
   g_clear_object (&circle_pixbuf);
 }