PdfSharpCore.Utils.FontResolver code cleanup, refactor

[MichelMichels]

As I was working on the previous support for the local AppData fonts in Windows, I noticed that the code was a bit 'old'.
Things I changed:

• Swapped the static constructor with a default constructor and make use of a static _isInitialized field to check if another instance already has loaded the fonts
• Swapped the Dictionary<string, FontFamilyModel> with a List<FontFamilyModel> and make use of LINQ to lookup the fonts
• Extracted the struct FontFileInfo to its own file in PdfSharpCore.Utils
• Renaming of a lot of properties/fields
• Style fixes

There were no tests, so I couldn't check if this breaks something. I did do a quick check with the font families from the issue in PR #267 and this worked like expected.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 56.80000% with 54 lines in your changes missing coverage. Please review.

Project coverage is 26.63%. Comparing base (2828a90) to head (91c7cd5).
Report is 99 commits behind head on master.

Files with missing lines	Patch %	Lines
PdfSharpCore/Utils/FontResolver.cs	49.53%	46 Missing and 8 partials ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #268   +/-   ##
=======================================
  Coverage   26.62%   26.63%           
=======================================
  Files         238      239    +1     
  Lines       23336    23360   +24     
  Branches     2809     2811    +2     
=======================================
+ Hits         6214     6221    +7     
- Misses      16629    16643   +14     
- Partials      493      496    +3     

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