Polish OSB plugin

[Martchus]

See particular commit messages

[kraih]

Cleaner indeed, especially the use of Mojo::DOM for XML parsing.

[perlpunk]

LGTM, except the sort keys issue.

[andrii-suse]

I've tried this and Plugin UI looks really mature now, thank you!

[codecov]

Codecov Report

Merging #2333 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2333      +/-   ##
==========================================
- Coverage   86.94%   86.89%   -0.06%     
==========================================
  Files         168      168              
  Lines       11139    11139              
==========================================
- Hits         9685     9679       -6     
- Misses       1454     1460       +6

Impacted Files	Coverage Δ
...penQA/WebAPI/Plugin/ObsRsync/Controller/Folders.pm	100% <100%> (ø)	⬆️
lib/OpenQA/WebAPI/Plugin/ObsRsync.pm	96.66% <100%> (ø)	⬆️
lib/OpenQA/Worker/WebUIConnection.pm	78.09% <0%> (-2.86%)	⬇️

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