[New Feature] PdfPageBuilder: Allow to copy pages from another document

[InusualZ]

This is a very naive implementation, because if you copy a lot of pages from a given document you would have duplicated resources. But, I would like to get some thoughts on this implementation.

I was surprise by the amount of code that this feature needed to be implemented. I was expecting a lot more. Even so, there is some duplicated code in two parts:

• Copying Token This piece of code keep reappearing, I really don't like it.
• Resource Management I think this piece of code and the one that write the resources to the stream can be transformed into a nice interface behind a read only interface and mutable interface, kind of like IReadOnlyList<> and IList<>, but IReadOnlyResourceStore and IResourceStore. I would like yours thought about that

Anyway, This code have some TODO I would like to address them, but I don't know If you even want the feature in it's current path of implementation, so maybe the effort to implement them are not worth.

If you have any suggestions, thought, anything feel free to comment.

[codecov-io]

Codecov Report

Merging #250 (7126564) into master (237fd96) will decrease coverage by 0.01%.
The diff coverage is 64.51%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
- Coverage   64.53%   64.51%   -0.02%     
==========================================
  Files         600      600              
  Lines       39725    39921     +196     
==========================================
+ Hits        25637    25756     +119     
- Misses      14088    14165      +77     

Impacted Files	Coverage Δ
src/UglyToad.PdfPig/Content/Page.cs	87.50% <ø> (+3.12%)	⬆️
src/UglyToad.PdfPig/Writer/PdfPageBuilder.cs	69.44% <51.14%> (-13.62%)	⬇️
src/UglyToad.PdfPig/Writer/PdfDocumentBuilder.cs	81.86% <95.94%> (+2.93%)	⬆️
src/UglyToad.PdfPig/Parser/PageFactory.cs	81.08% <0.00%> (-1.36%)	⬇️
src/UglyToad.PdfPig/Content/PageContent.cs	97.14% <0.00%> (+2.85%)	⬆️
...Operations/TextShowing/ShowTextsWithPositioning.cs	85.29% <0.00%> (+8.82%)	⬆️
.../MarkedContent/BeginMarkedContentWithProperties.cs	96.77% <0.00%> (+9.67%)	⬆️

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[EliotJones]

This looks like a useful feature to have, I'm just uncertain what the behaviour looks like for the copying a page's resource dictionary where the dictionary is using inherited properties, e.g. fonts, rotation or mediabox are set by the page trees pages node rather than at the page level. I think we can find out later and just start with this approach for now 👍

For the questions you raise in the opening PR comment I think we're fine taking the current approach for now, I'd avoid trying to derive the abstractions prematurely so I'm fine with doing things in a maybe less efficient manner to begin with.