PDF export: make embedding fonts optional

[Thomas Neumann]

Inkscape used to have an optional to embed fonts, with the default to off. Most users probably wanted to embed their fonts (and complained about missing fonts), so the option was removed and now all fonts are embedded when exporting to PDF.

However it is sometimes really useful to use un-embedded fonts. For example when including multiple SVG images in a paper, embedding fonts will result in duplicate copies of the fonts, which dominates the total PDF size. The fonts can then be embedded later to get a self contained PDF for the whole document.

Therefore I would like to have the option back, but with the default to embed fonts. Thus most users will be happy with the default settings, but users who need this feature can enable it manually.