use modern imgui modifier key names

[jemand771]

Summary

Switches over from ImGuiKey_ModXXX (deprecated in 1.89) to ImGuiMod_XXX. This should be backwards compatible (i.e. building with older imgui still works)

Motivation / Problem

Building with imgui >= 1.92.6

Changes / Key Changes

• ImGuiKey_ModXXX -> ImGuiMod_XXX

Testing

I sadly couldn't get pixi and/or this project's build tools set up, but it does build as the corresponding package in nixpkgs (which I'm trying to fixup in the first place)

Breaking Changes

none

Related Issues / PRs (backports)

• also see Support ImGui 1.91.5 and greater #1872
• same PR but based on release-6.16: use modern imgui modifier key names (6.16 backport) #2526 (files were renamed since then)

---

Checklist

• Milestone set (DART 7.0 for main, DART 6.16.x for release-6.16)
• CHANGELOG.md updated if required
• Add unit tests for new functionality
• Document new methods and classes
• Add Python bindings (dartpy) if applicable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.04%. Comparing base (81f98c5) to head (9296574).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2525   +/-   ##
=======================================
  Coverage   90.04%   90.04%           
=======================================
  Files         433      433           
  Lines       38390    38390           
  Branches     5311     5310    -1     
=======================================
  Hits        34570    34570           
  Misses       3820     3820           

Flag	Coverage Δ
unittests	90.04% <ø> (ø)

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