GUI v2: Document conversion interface

[apyrgio]

Main issue: #885

Update the window we show to the user when a document is being converted:

• Remove the Dangerzone logo and version
• Add "+ document" button, which should open the file browser window
• Add an explanation for what the user can do in this window
• Add a zebra row for each document (GUI v2: Document row #891)
• Have a visual cue that users can drag-and-drop documents to the list
  • ❗ We don't have a design for this
• Update the list when new documents are added (currently the user needs to close the application)
• Add the new footer (GUI v2: Footer #886)

Design material

Proposed conversion interface

Current conversion interface