set an image as main wikidata image

[nylki]

• via toolbar and context menu of an image
• potentially limit only show when navigated via a category
• when replacing a existing wikidata image, show before/after
• only show wikidata items that the image actually depicts already or is in commons category
• if only in commons category but not depicted: add depict statement to image
• show tip is TipKit, if an item has no main image yet, to introduce the feature to users