- fix "dereference after null check" error introduced with e6f3bc22 (mr
!455)
- fix "uninitialized member" error introduced with 99edb6c4 (mr !420)

https://coverity.cern.ch/reports.htm#v13572/p10041/fileInstanceId=763878155&defectInstanceId=396360325&mergedDefectId=98200

https://coverity.cern.ch/reports.htm#v13572/p10041/fileInstanceId=763879735&defectInstanceId=396360581&mergedDefectId=98201

This reverts merge request !636

and add a test which checks the ordering of EventIDBase

* mr-639:
  Remove deprecation warning from ExtraInputs and ExtraOutputs properties

* mr-638:
  Fix warnings seen when compiling with gcc8.

* mr-636:
  GaudiProjectConfig: Fix generated __init__ files to handle symlinks.

Closes #15

- change Gitlab-CI Docker image to latest LHCb one
- prefer host-binary-tag from PATH
  (see https://gitlab.cern.ch/lhcb-core/LbPlatformUtils/)
- use xenv from PATH or get it from the Git repository
  (see https://gitlab.cern.ch/gaudi/xenv)

if taken from Git it's also distributed in the installation directory

otherwise fall back on the provided get_host_binary_tag.py

partial overlap with !574

add target `test_headers_build` to test build of headers