SamplingBar: Improved event loop protection
Updated the sample rate selector when the config is changed
Added DevInst pointer to Signal
Removed _session.get_device calls from LogicSignal
Removed SigSession from Trace
Moved all sr_probe modification into pv::DevInst
SweepTimingWidget: When the exact value isn't available try to set a close match
Notify that config change when probes are enabled/disabled

This fixes 296