Modular Documentation Build in Coin

It would be glorious to have documentation built in Coin whenever a module is being built.
That doesn't come without problems though:

• qttools is needed
• we need index files of other modules

The plan (not perfect, but hey!):

• grab qdoc and a few other files from a previous build of qt5.git
  • bootstrapping this is a problem, probably they need to fall back to a build of the artifacts that does not require documentation and thus succeeds.
• as a first step upload all index files (work in progress by Frederik is there)
• then download all index files from previous qt5 builds
• then run qdoc on them
  • save the generated docs
  • log qdoc warnings to send them to gerrit (and maybe make it blocking one day)