Reorganize files and refactor code to make GaudiAlg optional

- move Gaudi::Funtional to GaudiFunctional subdir
  - headers are now under "Gaudi/Functional"
  - legacy GaudiAlg corresponding header redirect to the new ones
- replace GaudiAlgorithm with Algorithm wherever possible in tests and
  utilities
- add Gaudi::Sequencer basic implementation to replace GaudiSequencer
- move GaudiAlg specific tests, examples, dictionaries and Python modules
  to GaudiAlg subdir
- add CMake option GAUDI_ENABLE_GAUDIALG to allow skipping GaudiAlg
  (default to true)
- minor fixes and clean ups