This addresses #6543 by adding an option to prevent fonts such as Palatino and Times to automatically adapt the math font (IOW it lets you load the text font only for a bunch of fonts where this is easily possible).
Furthermore it adds an interface to select a specific math font, which is defined in latexfonts. Currently, this is only euler (the only one I know), but if there are other math-only tex fonts, they can be added easily (but note that this changes the file format).
Non-TeX math fonts are not yet supported. Eventually, unicode-math support can use the existing UI, but this is not on my agenda.
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