Improve the usability of the 'eval-on-submit' toggle button:
- Add a placeholder text that can be shown instead of the input field. This
can be used to describe what the evaluated Python code will do. In the
case of the 'Simple Blender Render' job, this is set to 'Scene frame
range'.
- Change the way in which the job type has to declare this, both for
clarity and to add the extra placeholder string
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