Streamline building IV

[Croydon]

This

• makes various ecosystem updates to the CI infrastructure
• most importantly: thanks to a lot of huge developments in the Conan ecosystem, we can finally get rid if ci-prebuilds and all related logic
  • this includes custom pre-processor logic
  • this also includes all Conan handling logic which isn't strictly for consuming packages
  • and a lot more....

I'm fully aware that the builds are currently broken and I'm tracking this at https://github.com/bincrafters/community/issues/604 as it is almost certainly not a problem in Inexor.

However, the current master is broken as well (see e.g. https://travis-ci.org/inexorgame/inexor-core/builds/477245258; [this particular issue is due to an old Conan version, but even if you would upgrade this you would run into the very same issue this pull requests displays]), so it makes nothing worse but makes it easier to eventually fix it (in fact, if it is really just an issue in protoc_installer as I believe it is that it will be "automatically fixed, as soon as this Conan recipe gets fixed. That Conan recipe updates are breaking retrospectively builds shouldn't happen anymore as soon as these packages are in conan-center, which is heavily work in progress for Protobuf and Protoc. It's just damn complicated... Also, Conan will soon introduce revisions as a feature which will prevent something like this in the future as well).

I'm also fully away that most people aren't interested right now to work on Core anymore due to all the elementary refactoring and redesigns going on, but I would really like to unwind ci-prebuilds sustainable.

Please merge this.

[a-teammate]

looks much cleaner :) :+1:

[Croydon]

This can be merged now.

As I am not an administrator anymore, I can't ignore the failing status checks (reasoning explained above) and can't merge myself anymore in this case.