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 (#891)
• [ ] Have a visual cue that users can drag-and-drop documents to the list
  • :exclamation: 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 (#886)

Design material

Proposed conversion interface

Current conversion interface