Use local socket connection for QML debugger

We have a local socket connection that the QML profiler already uses to connect to the application when running locally. There are several benefits to this:

1. We save the TCP overhead
2. The connection works the other way around, so that we don't have to wait for the application to open a server and coordinate the port selection.
3. Using the proper abstraction would allow us to add more connection mechanisms, without huge changes to the debugger code.

We should use QmlDebugConnectionManager also for the debugger.