Cleanup version stuff

* Cherry-pick @elen4's commit aa52a96 on yarp-2.4 branch
* Rename YARP_GENERIC_VERSION to YARP_VERSION and define it only once
* Add YARP_VERSION_STRING
* Remove YARP_VERSION_MODIFIER
* Better handling of YARP_VERSION_TWEAK
* Clean old stuff
* Move CPack stuff in YarpPackage.cmake