Add Melanosome Membrane

[HexapodPhilosopher]

Brief Description of What This PR Does

Replaces Melanosome model with a membrane-bound one.
Adjusts some text.

Related Issues

closes #5861

Progress Checklist

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• PR author has checked that this PR works as intended and doesn't
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  (this is important as to not waste the time of Thrive team
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• Initial code review passed (this and further items should not be checked by the PR author)
• Functionality is confirmed working by another person (see above checklist link)
• Final code review is passed and code conforms to the
  styleguide.

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[hhyyrylainen]

Thanks for the improved model, it looks good.
I'm not fully sure about the text changes, but let's just go with them (it's probably fine for the radioactive chunk to have a slightly longer name, probably no one will pay any attention to that change).