HV-819 Using more generic name "validated value handler" for type and member names

* Renaming API/SPI packages, types and members
* Adding package-info.java
* Improving documentation, marking API/SPI as experimental
HV-819 Incorporating further review remarks
HV-819 Making validated value unwrappers configurable via property
HV-819 Making member of ValidatorFactoryImpl immutable
HV-819 Updating reference guide; Adding new packages to OSGi manifest
HV-645 Unifying ConstraintLocation sub-types into one single class
HV-645 Making sure ElementDescriptor#getType() exposes the complete generic type
HV-645 Getting rid of elementType field in ConstraintLocation
HV-645 Making semantics of ConstraintLocation members more explicit