Extend Jenkins Build Pipeline

[aschaeffer]

• Added generation of the API documentation using Doxygen
• Added archiving artifacts
  • Currently bin/inexor, bin/inexor.exe, bin/RPCTreeData-inexor.proto
  • At a later point we can archive generated snap packages, debian packages or MSI installers
• Splitted building into two separated steps: fetching dependencies and build Inexor Core itself
• Use two cores for building Inexor Core

[aschaeffer]

API Docs for Inexor Flex

https://ci.inexor.org/job/inexor/job/flex/job/master/Inexor_Flex_API_Documentation/

API Docs for Inexor Core

https://ci.inexor.org/job/inexor/job/code/job/aschaeffer%252Fjenkins%252Fdoxygen/Inexor_Core_API_Documentation/

[Croydon]

All of that can be run in Travis is well. Please slow down a bit with Jenkins till I have my docker enviornment ready. Like I said I would like to revisit our CI requirments and what of them can be best done in Travis/AppVeyor/Jenkins. My setup is finally in pretty good shape, but I need a little bit more time. I just opened a dozen of pull requests for Conan/Conan packages. All this work is related.