preamble: move pygdbmi import to qml tracer and make it optional

while technically cool to have qml tracing is a bit of a niche feature now that kcrash defaults to post-mortem debugging instead of just-in-time debugging. since qml tracing requires a running process to run functions against we can't trace things now. plus, I've never seen a bug report with a trace anyway, so clearly it wasn't reliable one way or another. one less dependency people can miss to have the preamble fall over.