Flesh out deprecation messages for Config::{is,set}Default

Also, simplify the shim in ::isDefault
Remove toBe parens
:lipstick:
Store all settings in the scoped property store

There are a few failing specs, but I think they can all be addressed by
having a `source` for all properties and using it to target only the
user settings in various cases.
Fix Config::save
Fix Config::isDefault
Fix config event subscription methods

Signed-off-by: Nathan Sobo <nathan@github.com>