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.