splash image for windows portable application

[catdad]

Currently, the Windows portable application opens really slowly. It takes approximately 20 seconds, and users can't be expected to wait that long without any update showing.

I tested using asar, and that did not speed up the portable extraction time.

electron-builder has a splash screen requested feature (https://github.com/electron-userland/electron-builder/issues/2548) that I worked on a PR for (https://github.com/electron-userland/electron-builder/pull/4425), though I am blocked for now unless I can figure out another way to show progress during the unpacking (or a different packager altogether).

[catdad]

Splash images work now and have been working for a while (I use one here already). I just dropped the ball. Really, this is now just blocked by #303 for creating an image.