GUI v2: Document pop-up

[apyrgio]

Main issue: #885

Add a pop-up whenever a new document is added to dangerzone, either via drag-and-drop, or from the + document button. This pop-up should contain:

• The number of documents that were selected, or the document's name
• Simple settings for the document conversion that the user can tweak or leave as is
  • Whether to OCR the document
  • OCR language
• A message to the user about what will happen to the original document, and how the new document will be named (see #427).

Design material

Document pop-up

(ignore the "More settings" section)

Another idea for user message

(taken from #427 (comment), ignore the design difference)