Creator chapter hierarchy

[adrg]

• Add multi-level chapter hierarchy.
• Remove redundant subchapter component.

Resolves #353

[codecov]

Codecov Report

Merging #360 into v3 will increase coverage by 5.38%.
The diff coverage is 94.44%.

@@            Coverage Diff            @@
##              v3     #360      +/-   ##
=========================================
+ Coverage   55.2%   60.58%   +5.38%     
=========================================
  Files        152      151       -1     
  Lines      27301    27218      -83     
=========================================
+ Hits       15072    16491    +1419     
+ Misses     10375    10341      -34     
+ Partials    1854      386    -1468

Impacted Files	Coverage Δ
pdf/creator/creator.go	75.52% <100%> (+4.83%)	⬆️
pdf/creator/chapters.go	79.64% <94.11%> (+16.75%)	⬆️
pdf/annotator/field_appearance.go	11.5% <0%> (+0.62%)	⬆️
pdf/creator/division.go	89.13% <0%> (+1.08%)	⬆️
pdf/model/outline.go	86.74% <0%> (+1.2%)	⬆️
pdf/creator/invoice.go	90.07% <0%> (+1.21%)	⬆️
pdf/model/sighandler/sighandler_rsa_sha1.go	6.75% <0%> (+1.35%)	⬆️
pdf/annotator/form_field.go	15% <0%> (+1.42%)	⬆️
pdf/model/annotations.go	23.08% <0%> (+1.48%)	⬆️
pdf/creator/toc_line.go	75.39% <0%> (+1.58%)	⬆️
... and 95 more

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