Projects plugin: rudimentary support for an rtags client

Right now, rtags needs to be starte manually based on the file
compile_commands.json. For your project, run cmake with the
option -DCMAKE_EXPORT_COMPILE_COMMANDS=ON or add to CMakeLists.txt
  set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
Then, run the rtags daemon somewhere through rdm &. And then go
into the build folder where the compile_commands.json file is located
and tpye `rc -J`. Then, open Kate with the projects plugin and
you can follow the symbol under the cursor by pressing ALT+G.
This is just a test for now, so expect more to come...