Add ability to run external command on image

This can be useful for e.g. uploading to paste services without hardcoding any in flameshot itself. Another usecase could be to run some imagemagick filter on it.

This option should either use a temporary file or be part of an extended save process (e.g. Ctrl+Shift+s).

Start by reviewing the existing image save flow and how an extended save action could expose the captured image to an external command. Done means a user can invoke a configured command with the image, including for paste-service uploads or ImageMagick-style processing, without hardcoded services.

Written by the indexing model from the issue text.