Gets the "default value" of a key.
This is the value that would be read if reset were to be called on the key.
Note that this may be a different value than returned by get_default_value if the system administrator has provided a default value.
Comparing the return values of get_default_value and get_value is not sufficient for determining if a value has been set because the user may have explicitly set the value to something that happens to be equal to the default. The difference here is that if the default changes in the future, the user's key will still be set.
This function may be useful for adding an indication to a UI of what the default value was before the user set it.
It is a programmer error to give a
key that isn't contained in the schema for this.
a Settings object
the key to get the default value for
the default value