refactor: Change ObjectSelector to Selector

[hoxbro]

Let's see how it goes.

Went great 🥇

See holoviz/param#728

[codecov]

Codecov Report

Attention: Patch coverage is 98.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.50%. Comparing base (3acfadb) to head (68aebb4).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/ipython/widgets.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6444   +/-   ##
=======================================
  Coverage   88.50%   88.50%           
=======================================
  Files         323      323           
  Lines       68631    68631           
=======================================
  Hits        60741    60741           
  Misses       7890     7890           

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