Enable use of variable-width fonts for displaying symbols, in a way that does not conflict with fixed-width-space-based indentation. Notes: - Customize `propfont-mixed-inhibit-regexes' to forbid some symbols from being shown with proportional fonts. See also `propfont-mixed-inhibit-function', and `propfont-mixed-inhibit-faces'. - It is probably necessary to adjust the face `variable-pitch', so that the proportional font looks good and is the correct size to match the default font.