[Request] Improve UI for sending/receiving (multiple) file(s)

[schlagmichdoch]

I have some issues with the current implementation when sending and receiving multiple files:

• The 'Ask to save each file before downloading' button is never shown again once it is unchecked
• When receiving mutliple files we do not know how many files are sent and how big they are
• When sending multiple files we do not know whether all files have been sent successfully or what the overall status for sending files is
• Files are sent and received without the user accepting the transfer which presents a security issue

I would love an implementation that is similar to Apples AirDrop:

• Show a Dialog with a possibility to "Accept" or "Decline" the file to be send and only start sending afterwords
  • First, only a header is sent which includes: number of files and maybe the overall size to the receiving peer
  • if only one File is sent and it is an image, add an image thumbnail to show the file
    • Thumbnail dimensions:
      • max-size: 64kb (size of one chunk)
      • max-width: 352px (max-size of preview #559 )
      • probably reducing the image width to 352px should be enough to get below 64kb
• Only after the receiving peer accepts the file(s) they are transferred one by one.
• (Transfer status is shown dependent on the files already sent)
• 'transfer-complete' is only sent when all files are transferred successfully
• When all files are transferred successfully do not show a ReceiveDialog anymore (as we already did before transferring) but immediately start the download of all files

I think this would increase snapdrops security and intuitivity but needs some work.