Added some missing fromUtf8s

This fixes the issues mentioned in #307
Do not attempt to decode when required probes have not been specified

This fixes 204
DecodeTrace: Draw the error over the unresolved period
Wrapped sr_dev_inst in a class: pv::DevInst
Fix some disappearing annotations
Moved config getting/setting into DevInst
Added DevInst::config_changed signal
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
Notify that config change when probes are enabled/disabled