Support icon shorthand for all widgets and menus

[philippjfr]

This PR adds first-class support for Material icons across all widget labels, Select options, and Menu items in panel-material-ui.

Icons can now be embedded directly in strings using the following syntax:

:material/zoom@size=large,color=warning:

• material/zoom selects the Material icon
• The @… section is optional
• Supported modifiers include size, color, and other Material icon props

Why this matters

• Enables richer, more expressive UIs without custom renderers
• Keeps APIs declarative and string-based
• Aligns icon usage across Panel and Material-UI components

Backwards compatibility

• Fully backwards compatible
• Plain text labels continue to work unchanged

This significantly improves visual affordances while keeping the API simple and uniform.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.18%. Comparing base (1d7d7b8) to head (30ce884).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #569   +/-   ##
=======================================
  Coverage   81.18%   81.18%           
=======================================
  Files          31       31           
  Lines        3168     3168           
=======================================
  Hits         2572     2572           
  Misses        596      596           

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