Sets key in this to value.
It is a programmer error to give a key that isn't contained in the schema for this or for
value to have the incorrect type, per the schema.
If value is floating then this function consumes the reference.
| this |
a Settings object |
| key |
the name of the key to set |
| value |
a Variant of the correct type |
|
true if setting the key succeeded, false if the key was not writable |