Add alt-text

[sboosali]

This could default to the card text with the filename of the art image, but should be editable (another box? in the pop up during image import?), so the user can provide art descriptions (for HTML).

Relatedly, an option (disabled by default) to export a .txt alongside the .png during image renders, is a feature that would be very helpful for me (since it's something that I've been doing manually) tho if no one else would use it, I'll just work on it myself.