Add Creator support for outlines

[adrg]

Generate PDF outlines automatically for Chapter and Subchapter components in the Creator.
Also, adds a high level interface for creating outline trees in the model package, compatible with the
low level interface.

Resolves #87
Resolves #14

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This change is 

[codecov]

Codecov Report

Merging #321 into v3 will decrease coverage by 0.42%.
The diff coverage is 39.28%.

@@            Coverage Diff             @@
##               v3     #321      +/-   ##
==========================================
- Coverage   38.99%   38.56%   -0.43%     
==========================================
  Files         120      121       +1     
  Lines       22945    23081     +136     
==========================================
- Hits         8947     8901      -46     
- Misses      12743    12899     +156     
- Partials     1255     1281      +26

Impacted Files	Coverage Δ
pdf/model/outlines.go	0% <0%> (ø)	⬆️
pdf/model/outline.go	0% <0%> (ø)
pdf/creator/creator.go	70.09% <100%> (-5.56%)	⬇️
pdf/creator/chapters.go	62.88% <79.16%> (-3.79%)	⬇️
pdf/creator/subchapter.go	68.68% <79.31%> (-4.81%)	⬇️
pdf/creator/toc.go	85.32% <0%> (-8.26%)	⬇️
pdf/creator/toc_line.go	73.8% <0%> (-6.82%)	⬇️
pdf/creator/block.go	67.61% <0%> (-6.44%)	⬇️
pdf/creator/styled_paragraph.go	86.18% <0%> (-4.53%)	⬇️
... and 4 more

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[gunnsth-review]

Reviewed 5 of 5 files at r1.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @adrg)

a discussion (no related file):
Looks great. Couple of suggestions and questions.

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pdf/model/outline.go, line 112 at r1 (raw file):

// OutlineItem represents a PDF outline item dictionary (Table 153 - pp. 376 - 377).
type OutlineItem struct {

Good approach to wrap the PdfOutlineItem in an OutlineItem (higher level), separating the creation function and the logic.
Can this also be used when reading the outlines, e.g. getting a list of all outlines?
Might be good to add a comment that this is intended for creation use.

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pdf/model/outlines.go, line 50 at r1 (raw file):

func NewPdfOutline() *PdfOutline {
	outline := &PdfOutline{
		primitive: &PdfIndirectObject{

More concise syntax: core.MakeIndirect(core.MakeDict())

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pdf/model/outlines.go, line 69 at r1 (raw file):

func NewPdfOutlineItem() *PdfOutlineItem {
	outlineItem := &PdfOutlineItem{
		primitive: &PdfIndirectObject{

same here

[adrg]

Reviewable status: 3 of 5 files reviewed, 4 unresolved discussions (waiting on @adrg and @gunnsth-review)

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pdf/model/outline.go, line 112 at r1 (raw file):

Previously, gunnsth-review wrote…

Good approach to wrap the PdfOutlineItem in an OutlineItem (higher level), separating the creation function and the logic.
Can this also be used when reading the outlines, e.g. getting a list of all outlines?
Might be good to add a comment that this is intended for creation use.

I added a comment on the Outline struct specifying that currently it can be used only to build outlines. The outline object could be used to also read the outlines from a PDF file.
I think we should do this in another pull request though and make this one about the Creator. The interface for the high-level outlines could contain a method for reading outlines from a PDF file, one for printing them in user friendly format. Maybe merging and splitting as well.

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pdf/model/outlines.go, line 50 at r1 (raw file):

Previously, gunnsth-review wrote…

More concise syntax: core.MakeIndirect(core.MakeDict())

Done.

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pdf/model/outlines.go, line 69 at r1 (raw file):

Previously, gunnsth-review wrote…

same here

Done.

[gunnsth-review]

Reviewed 2 of 2 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @adrg)

[gunnsth-review]

Reviewable status: complete! all files reviewed, all discussions resolved