Gets the value that is stored at key in this.
A convenience function that combines get_value with @get.
It is a programmer error to give a key that isn't contained in the schema for this or for
the VariantType of format to mismatch the type given in the
schema.
| this |
a Settings object |
| key |
the key to get the value for |
| format |
a Variant format string |
| ... |
arguments as per |