Image / Image Picker - Add an option to replace an image using drag-and-drop

[JaneSjs]

T18153 - Drag and Drop in "Image" vs. "Image Picker" https://surveyjs.answerdesk.io/internal/ticket/details/T18153

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Users can drag-and-drop an image and upload the image for an empty Image question / Image picker item. However, if an image exists, the drag-and-drop won't replace an image. A survey creator should click a file upload button to upload a new image and replace an existing one.

Introduce an option to use drag-and-drop to replace an existing image for Image and Image Picker questions.