Validator should verify contents of appicon.ico

[GordCaswell]

At present, appicon.ico validation does not happen. It is validated if it exists, but not if the correct icon sizes are in place. It will be necessary to error if any of 16,32 & 48 px 8-bit & 32-bit sizes do not exist, and warn if 256px 32-bit does not exist.

I am looking into using PIL and a plugin for it to do so.