Trace reference selection - only available when trace is on.
this could be faster, by giving freedom to choose no matter whether trace is visible (1. select, 2. show)
if bad trace layout is currently selected each switching it on first - takes extra time (worst case scenario trace being whole layout with multiple drawings on it). Takes time even before having chance to cancel layout update
Interestingly, trace off: "choose reference" is not grayed out but, sub-menu content list is 😉 it's a bug
