This is a follow up on the previous attempt #106061 and implements alias support for simple and list settings on top of fallbacks. The alias behavior of a fallback is enabled by adding Property.Alias.

Fallback settings (as is today) are not well-suited when migrating a setting from one key to another. The problem is that update consumers are not properly triggered if both the old (fallback) and new setting are present causing changes to be silently ignored if the old version is used.

Alias fallbacks behave similar to fallbacks with some few differences:

• updates to the alias are copied to the alias target in addition, so that update consumers are triggered properly.
• Setting.exits doesn't consider fallbacks, but considers aliases which impacts the behavior of Setting.get using primary and secondary settings.