Cleanup popup

[ahuang11]

Changes so that internally, it will not touch existing_pane/popup_pane's param. That is up to the developer/user, so if it's not visible, the popup pane will not show, primarily for holoviz/holonote#101

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 87.57%. Comparing base (eb7ae9d) to head (40d59e0).
Report is 4 commits behind head on main.

Files	Patch %	Lines
holoviews/plotting/bokeh/callbacks.py	0.00%	4 Missing ⚠️
holoviews/tests/ui/bokeh/test_callback.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6207      +/-   ##
==========================================
- Coverage   88.12%   87.57%   -0.56%     
==========================================
  Files         319      321       +2     
  Lines       67221    67289      +68     
==========================================
- Hits        59241    58929     -312     
- Misses       7980     8360     +380     

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[ahuang11]

I believe these changes make the experience better:

• popup_pane visibility is controlled by the user; if it's visible=False then popup pane won't show
• Setting a popup_pane with tooltips to have a position to NaN resulted in the tooltips still appearing at the corner of the page (notice it the top right). Changing it to visible = false makes that not happen.

The only thing I couldn't figure out how to do is de-duplicate the same layout across multiple streams, e.g. TapStream and BoxSelect both popup _layout. Not sure how to check if the ID of this layout already exists in the document; if so, delete the previous one.

[hoxbro]

I can't judge if this is a better approach than the previous approach.

If your opinion is that it is, you can merge the PR unless @philippjfr has a hard objection.

[philippjfr]

I'm happy with this approach.