MudMenu: Disable text selection in activator

[danielchalmers]

Description

After #8598 the chip example is now selectable despite being clickable, because the clickable-ness is done on the menu end.
This is especially noticeable when using touch to hold the activator for right clicking.

This resolves it and disables all selection for anything inside the activator, not just chips, to make it a true button.

How Has This Been Tested?

visually

Type of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or improvement to the website or code docs)

video1.mp4

Checklist

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.06%. Comparing base (28bc599) to head (df44f80).
Report is 64 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8719      +/-   ##
==========================================
+ Coverage   89.82%   90.06%   +0.23%     
==========================================
  Files         412      418       +6     
  Lines       11878    12050     +172     
  Branches     2364     2366       +2     
==========================================
+ Hits        10670    10853     +183     
+ Misses        681      661      -20     
- Partials      527      536       +9     

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[henon]

Thanks Daniel