set_boolean
Description:
Sets key in this to value.
A convenience variant of @set for booleans.
It is a programmer error to give a key that isn't specified as having a boolean type in the schema for 
        this.
Parameters:
| this | a Settings object | 
| key | the name of the key to set | 
| value | the value to set it to | 
Returns:
| true if setting the key succeeded, false if the key was not writable |