/* add_trace adds a trace into the current traceset. It first displays a
* directory selection dialogue to let user choose a trace, then recreates
* tracset_context, and redraws all the viewer of the current tab
/* add_trace adds a trace into the current traceset. It first displays a
* directory selection dialogue to let user choose a trace, then recreates
* tracset_context, and redraws all the viewer of the current tab