Progress indication when writing multiple frames

Processing all frames of an animated image file (i.e. GIF, APNG, WebP) can take a while, and there's no indication of any sort for progress there, nor is there any way to keep track of it. Thus, I suggest adding to save_all implementation at least one of the following:

• a log output (either at INFO or DEBUG level) after every frame (e.g. the way it's done by convert -verbose)
• an (optional) custom progress tracking callback parameter (which should be given the current frame index and possibly total n_frames & filename of the first file as well); this can be used for customizing log output or for GUI progress indication
  • log only when no progress tracking callback is provided?
  • alternatively, if there's a callback and it returns a string, print it out as logging output?

Start by locating the save_all implementation for animated image files and review how GIF, APNG, and WebP frames are written. Compare the proposed logging and callback approaches, including the available frame and filename information. Done means progress can be tracked during multi-frame saves with behavior covered by the relevant tests.