In show_file, use os.remove to remove temporary images

[radarhere]

Rather than using rm inside a subprocess shell command, this PR switches to using subprocess without shell to open the image and then os.remove. This should resolve any path problems.

But there is a twist on macOS - Pillow sleeps before removing the image, and we presumably don't want to make our Python script hang for 20 seconds. Instead, this PR uses subprocess to start a Python process that sleeps and removes the file afterwards.

And since that makes sleeping before removing the file nicer, this PR adds sleep for xdg-open, instead of #5950, to help #5945 and #5968