format: Add API to query a property's type

format: Add API to query a property's type

Since a given property can be single-valued, an array or (in the case of
ints) a range, clients need an API to figure out what type of value a
property holds. This adds such an API. The actual property type
enumeration is kept in the PA_PROP_* namespace and not the
PA_FORMAT_INFO* namespace so that it can later be reused for properties
generically if required.