Add custom icons for different file types

[JacobTheDev]

Currently, all file types associated with Affinity Photo or Affinity Designer share the same icon (that of it's associated program). It'd be nice if each file type (at least the ones natively supported) where to get its own unique icon. For example, perhaps the SVG file type could be a file with the letters "SVG" superimposed on the center, with the Affinity Designer logo in the bottom corner. See the attached PNG for a rough mockup.