fix (CName): Validate setter on Name

[jafin]

fix (Cname): Setting name now validates value rather than previous set private name.
Credits original author @jeanbern empira/PDFsharp#86

[codecov-commenter]

Codecov Report

Merging #261 (a99c642) into master (14080aa) will increase coverage by 0.07%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
+ Coverage   26.37%   26.45%   +0.07%     
==========================================
  Files         238      238              
  Lines       23329    23329              
  Branches     2808     2808              
==========================================
+ Hits         6154     6172      +18     
+ Misses      16684    16667      -17     
+ Partials      491      490       -1     

Impacted Files	Coverage Δ
PdfSharpCore/Pdf.Content.Objects/CObjects.cs	5.33% <61.53%> (+5.33%)	⬆️
PdfSharpCore/root/PSSR.cs	5.93% <0.00%> (+0.84%)	⬆️
PdfSharpCore/Fonts/FontFactory.cs	35.25% <0.00%> (+1.28%)	⬆️

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