Use subprocess with CREATE_NO_WINDOW flag in ImageShow WindowsViewer

[radarhere]

Resolves #7789

The user has found that calling using ImageShow from within a .pyw file on Windows causes a console to appear, as a result of calling os.system from Viewer. https://github.com/python-pillow/Pillow/blob/6782a07b8e404271dbd2e5ddf5fbb93e575ed2bc/src/PIL/ImageShow.py#L121

The user has suggested calling subprocess with the CREATE_NO_WINDOW flag to fix this. https://stackoverflow.com/questions/58943289 / https://stackoverflow.com/a/7006424/4093019 agree with this solution.

Because subprocess.CREATE_NO_WINDOW only exists on Windows, I've added a dedicated WindowsViewer show_file() method for this.