Dbg: Fixed duplicated updateInfo() calls and showing vars.

Connecting "currentIndexChanged" qt combo signal was calling multiple
times the routines to update the view (when a combo item was added, when
.clear() was called, etc) -- hence how terribly slow stepping was. I did
a dirty fix by exposing a custom onUpdate() on the StackCombo class and
checking if it is info is being updated before notifying external
entities that it changed.