We now handle 3 different types of property value types of setters
differently:

* For reference arg, we convert from `&Value` (so `TryFrom<&Value<'_>>` is
  required).

* For argument type with lifetimes, we convert from `Value` (so
  `TryFrom<Value<'_>>` is required).

* For all other arg types, we convert the passed value to `OwnedValue`
  first and then pass it as `Value` (so `TryFrom<Value<'static>>` is
  required). This implies possible implicit allocation/cloning but what
  can we do. 🤷

Fixes #248.