Allow web favicon to be built from a different source image file

[galenwarren]

This regards this issue.

It would be useful to be able to use a different source image to generate the favicon vs. the image icons for Flutter web. Two reasons:

• Favicons with transparent backgrounds work well, in general, but transparent backgrounds don't work well for icons for PWA apps (a background gets assigned which might be white, might be black, etc.)

• PWA icons have the same issue that Android icons do, i.e. adaptive icons need to limit content to the "safe zone" so that the icons can be cropped as needed. Favicons don't need this extra padding and really can't afford to have it, given how small they are to begin with.

This PR adds an image_path_favicon option to the config to allow one to override the source icon used to generate the web favicon. This would allow one to specify a non-padded icon with a transparent background to use for favicon generation and a padded icon with a solid-color background for other image generation.