[image-decoder] Convert to C++STL

[sharder996]

Part 2 of removing Qt one piece at a time. This time in the XzImageDecoder class.

This PR replaces a few usages of mp::Path with std::filesystem::path and QFile with file streams.

Eliminates one more leaf node of the Qt network and pushes it up one more function call.

[codecov]

Codecov Report

❌ Patch coverage is 7.14286% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.17%. Comparing base (2cbfc9d) to head (45a65f0).
⚠️ Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
src/xz_decoder/xz_image_decoder.cpp	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4571      +/-   ##
==========================================
- Coverage   87.18%   87.17%   -0.01%     
==========================================
  Files         247      247              
  Lines       14273    14275       +2     
==========================================
  Hits        12443    12443              
- Misses       1830     1832       +2     

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[jimporter]

The changes themselves all look good, though I'm a little worried since there's no test coverage for this code (at least not reported to Codecov). Would it be feasible to add some tests here?