Default page master layouts are now protected

Two improvements for users who customize default page masters:

Protected page master layouts:
Default slots on Displayr's standard page masters can no longer be deleted. This means that automated features like Add commentary won't break your custom designs. If a default page master has already been customized, you'll see a warning with a Fix button that restores it, while retaining your customized page master.
Layout change notifications:
When Add commentary or Add auto-updating commentary changes your page's page master, you'll now see a notification letting you know.