Add extra fields option for image and imagelist fields

[xiaohutai]

For both images and imagelists, you can now add an extra property extra:

extra:
    title:
        label: Titel
        placeholder: Placeholder
    caption:
        label: Caption
    teaser:
        label: Teaser
        placeholder: De teaser in het Nederlands
    teaser_en:
        label: Teaser (EN)
        placeholder: De teaser in het Engels

For now I have enabled label and placeholder, but since it's an key,value-array we can expand it if needed. (I initially had it as: attrib: [ foo, bar, baz ] to mimick Bolt 3).

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This seems to work for me. Somehow the solution looks way simpler than I thought. So please double-check if I haven't missed anything.

[bobdenotter]

Did two ninja-edits! Looks good, otherwise! 👍