notify is called by subclasses when the value of a metadata variable changes.
It emits the `changed` signal with the variable name as the detail
parameter. Set shadow = true if variable is shadowed by a property so that a notify signal
is emitted with the property name as the detail parameter.
| this | |
| pspec |
The ParamSpec used to declare the variable. |
| shadow |
The metadata variable shadows a property. |