Checks the "user value" of a key, if there is one.
The user value of a key is the last value that was set by the user.
After calling reset this function should always return null (assuming something is not wrong with the system configuration).
It is possible that get_value will return a different value than this function. This can happen in the case that the user set a value for a key that was subsequently locked down by the system administrator -- this function will return the user's old value.
This function may be useful for adding a "reset" option to a UI or for providing indication that a particular value has been changed.
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 user value for
the user's value, if set