Import/Export flows

[bertob]

Summary of the design discussion with @adz yesterday:

At the most basic level, USB support just means supporting importing/exporting files, so that's where we should start.

Possible flows:

• Import changes to a document you don't have yet locally (but you're invited to it)
  • "Import from File..." in primary menu
  • Choose file in file chooser
  • Show imported document in new window, with a toast "Imported from File"
• Import changes to the currently open document
  • "Import from File..." in primary menu
  • Choose file in file chooser
  • Dialog previewing the diff (and maybe eventually allowing to selectively apply only some changes or creating an alternate copy of the document?)
  • Show imported document in new window, with a toast "Imported from File"
• Import changes to another document you have locally
  • "Import from File..." in primary menu
  • Choose file in file chooser
  • Open a new window with the other document
  • Modal dialog on the new window, previewing the diff (and maybe eventually allowing to selectively apply only some changes or creating an alternate copy of the document?)
  • Show imported document in new window, with a toast "Imported from File"
• Export to file
  • File chooser save dialog