Add a unix domain socket connection mechanism for QML debugging and profiling

It would be good to have QML debugging and profiling (and preview) support without QtNetwork. You can build QML with -qml-no-network after all, which will retain most of the debugging and profiling support, except for the connection plugins. The TCP connection obviously doesn't work then. QLocalSocket unfortunately is also part of QtNetwork, so the local socket connection doesn't help either. We could, however, add a third connection plugin that wraps some other OS facility to transmit data between the processes.

There is ongoing work to use the C++ debugger as data channel, but this is far from finished and comes with its own dependency.