Implement AppBar component

[philippjfr]

Implements standalone AppBar component, making it possible to build Page like layouts manually.

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.11%. Comparing base (bfab387) to head (3fc59e5).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/panel_material_ui/template/base.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #680      +/-   ##
==========================================
+ Coverage   82.86%   83.11%   +0.25%     
==========================================
  Files          33       33              
  Lines        3799     3820      +21     
==========================================
+ Hits         3148     3175      +27     
+ Misses        651      645       -6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.