Add compilation database plugin

Opening an C++ file without an project leads to an subpar experience because our C++ support needs some build  information like include paths and compiler flags to provide correct highlighting, code completion etc.. But CMake based projects provide that information already in the  "compile_commands.json" file. We now can open it an provide a simple project view of the project.

In portale's words:
The ability to import and export (also in Qt Creator 4.8) compilation databases opens up Qt Creator to interoperate with a growing number of tools which support this standard.

• Import: Several build systems (which are not supported by Qt Creator) can export a compilation database. Qt Creator can now be used as C++ editor with all the bess an whistles
• Export: Any tool that accepts compilation databases (also non-clang-tools like https://www.sourcetrail.com/ ) can now be fed with any project from Qt Creator.
  An interesting overview of opportunities can be found here.