Dark NSIS installer wizard image
Matching the dark splash screen introducedin !91 (merged), here is a dark image for the Windows installer wizard too.
Since NSIS is rather sensitive w.r.t. image format, I followed the exact instructions here.
Test plan (not executed myself):
- Testing must be done on an actual Windows PC! Testing in Wine doesn't count.
- Start the installer programme.
- Check that the image is shown correctly along the left side of the first page of the wizard.