Exclusions for PDF font embedding

Migrated from https://bugs.launchpad.net/inkscape/+bug/1762898
From 2018-04-11

Summary:

For some well-known PostScript-named fonts, especially those in the ubiquitous "core13" (Times, Helvetica, Courier, Symbol) set, users may not be able to embed these fonts for availability or legal reasons. Inkscape should probably provide an option to reference such a font in PS-speak without embedding it. In addition, some replacement capabilities should be allowed for mapping from actual core13 implementations (e.g. URW Nimbus Sans) to the standarized core PS names (e.g. Helvetica).

Consider an SVG that uses Times New Roman and Arial in its text, being processed on a typical Linux desktop. For PNG export fontconfig will find the best local matches (Liberation Serif/Sans), but the replacements are undisirable and unnecessary in PDF: for the two core13 fonts, asking for "Times" and "Helvetica" will automatically make the PDF reader use their local copies! These local versions will provide a better match if viewed on a Mac or a Windows box; at worst they will give the same Liberation set on Linux.

(PDF uses a slightly different Core Font set: https://en.wikipedia.org/wiki/PostScript_fonts#Core_Font_Set. It's basically core13 + ITC Zapf Dingbats, which isn't that common...)