fix: Reuse dashboard tabs when reassigning the variable

[mattrunyon]

Fixes #1971

Tested with this code run 1 line at a time

import deephaven.ui as ui

a = ui.dashboard(ui.text("Hello"))

a = ui.dashboard(ui.text("World"))

a = ui.text("Hello")

a = ui.dashboard(ui.text("Hello"))

del a

Also tested dashboards still load in embed-widget. There was a race condition since the makeUseListenerFunction hook was using useEffect. This caused the event to emit between the event hub being set and the listener being added.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.44%. Comparing base (aad0aa6) to head (7f47596).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2243      +/-   ##
==========================================
+ Coverage   46.42%   46.44%   +0.01%     
==========================================
  Files         698      698              
  Lines       38750    38736      -14     
  Branches     9885     9885              
==========================================
- Hits        17990    17989       -1     
+ Misses      20706    20693      -13     
  Partials       54       54              

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