MarkerView: rework drawing to reduce flicker.

Addresses another part of #9841.
Reduce flickering in VariablesView.

Since the individual _AddNode() invocations notify their node additions,
NotifyTableModelReset() isn't really appropriate here after all, since
the net effect will be seeing all the nodes getting added, then removed
again, then re-added. Also fixes the fact that the variables wouldn't get
cleared when picking Run, until we stopped again.
Fix remaining flickering for #9841.

- Revert hrev45802. However, implement the same essential logic in
  the TeamWindow itself, and have it guard the StackTraceView,
  VariablesView, and step control buttons. This fixes flickering
  in between short steps since it prevents the intermediate clear
  from happening unnecessarily.
- Implement appropriate guards for the interim wait state.
Fix redrawing issue.

If we had a source code change that resulted in a new file that was shorter than
the entire view, the section below the last line wouldn't get repainted properly.