Notify about missing fonts when loading XCF
Also gets rid of missing fonts check in the PDF export plugin, since fonts are renamed internally, that checking logic relied on entirely on Pango, but pango can't tell which fonts are actually missing since we give it a generated font name and not the actual font name (see !1011 (merged) and !1110 (merged) for more details).
An edge case that's not covered is if some fonts in use are made inaccessible mid-session. IMO that's a rare case, and would complicate the code a lot more to accommodate it, so it's better to ignore it for the time being.
Closes: #1547
Edited by Idriss Fekir