Concepts: Gradients: Misc updates and corrections

• Explain the term "drawing tool" before using it as figure title
• Sort required Pencil dialog settings in the UI order
• Correct capitalization of mentioned settings
• Update C screenshot (missing additional "Blend Color Space" option) under "Color Options"
• Remove outdated localized screenshots (missing additional "Blend Color Space" option)
• Split screenshot into separate translatable and non-translatable files (because translators)
• Link to docs of mentioned gimp-toolbox-color-area (renamed for consistency, cf !201 (merged))
• List additional special options (first six instead of four gradients are special)
• Semantically turn the list of special options into a list (because readability)