fix: burger menu to also work with new core icons

[fsbraun]

Description

This PR fixes the burger menu to

1. work with icons provided by the core
2. variable width content
3. Only display if there are elements in the burger menu
4. CMS style (same as e.g., state indicator drop down or page tree dropdowns)

Example djangocms-admin-style

Example bare django admin

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Merging #323 (5ed516c) into master (1e8a48c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #323   +/-   ##
=======================================
  Coverage   90.53%   90.53%           
=======================================
  Files          68       68           
  Lines        2324     2324           
  Branches      314      314           
=======================================
  Hits         2104     2104           
  Misses        165      165           
  Partials       55       55           

Impacted Files	Coverage Δ
djangocms_versioning/admin.py	89.13% <ø> (ø)
djangocms_versioning/indicators.py	91.07% <100.00%> (ø)

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[marksweb]

LGTM