make format_str to work with special treatement for info label

[nico1080]

The toml file have option to concatenate text/info label in a single file with format_str.
Some info label have "special treatment" eg: artist, codec, acodec, full_codec etc.

The format_str option is not able to concatenate theses specials treatment.

Will it possible to add a syntax to allow that?

[mattblovell]

See third bullet in possible implementation ideas for Idea #16.

[mattblovell]

A distinction may be needed between "special treatments" or display callback functions that yield a string to display versus the new possibility (from Idea #20) that result with the in-progress image being manipulated (e.g., a codec image rendered). Only those functions that return a string lend themselves to format_str interpolation, naturally!

[mattblovell]

Should be possible with v1.22

Note that only string callback functions (i.e., those that are only passed enough information to do pure string manipulation) can be used as part of format_str interpolation. Having the drawing-capable "element" callback functions also be available would have required passing the image and draw objects to format_InfoLabels(), which didn't seem desirable.