Reduce PropertyInfo construction overhead

The private data is static and immutable, so it can be shared by all
instances. This significantly reduces the overhead of instantiating
a PropertyInfo for the most common cases. Most importantly, the
i18n of displayString is delayed, so e.g. `Property(...).name()` and
`PropertyInfo::fromName(...).property()` are faster by an order of
magnitude.

All public access functions are already const, and the public class
is pimpl'ed, so this change is API and ABI compatible.