GimpFontFactory, GimpFont: Improve font matching with old XCF files and fonts validity check

• Don't rely on the extension in file names to determine the validity of fonts.

• Retrieve the actual font files that pango would use for a given pango font description and use that to look for a matching font, this is because pango when given an unsupported font, will find the closest supported font but still report the unsupported font's name (e.g. the xcf file from #9220, reports the font to be Century Schoolbook L which is an unsupported Type1 font but actually it uses C059-Bold if it's available (a TTF font).

• Also fixes #11341.