value: fix parsing of explicit value casts
Since acdb4ce0 , parsing of the value for a property can use the pspec to determine what type a value should be casted to.
However, this broke the case where the value is explicitly casted to a type (eg <(float) 0.0>). In that situation, we want to respect the casting decision, and only use the pspec to perform "implicit" casts.
Edited by Mathieu Duponchelle