Improve File Transfer GUI

[peterstory]

Some "nice to haves:"

• Display a confirmation when a file is received
• The receive confirmation should offer to show the files. Also, it should be permanently dismissible.
• Increase window size on >100% scaled Linux displays (main window, file received modal, and file picker)
• Increase padding on Linux so buttons aren’t right at the end of the screen (e.g., especially the +/- files on the Send tab: they are hard to click since the window resize option is nearby)
• How we patch the file chooser is kludgy. Also, if the file chooser is open to a directory and files are added to that directory (e.g., a file is downloaded), then the file chooser doesn't update automatically. fsdialog might be better, but it doesn't scale properly on 4K screens.
• Only allow once instance of the application on Linux. This requires window-manager specific code, so I will keep the code in the user-study branch.
• Suppose the user is performing a series of transfers. If an error occurs, the "Received Files" dialogue should be closed if it was already open. Otherwise, it will look like the transfer succeeded. (Code is on the dev branch)
• Add a delay to the "Start Sending" button. Currently, if you click the button rapidly, you can trigger an error on the receiver.
• When the file transfer queue is changed, clear the progress bar. Otherwise, you cannot tell if you transferred the files already.
• Option to remove items from the queue after a successful transfer
• Disable buttons which don't make sense (e.g., - button when nothing is selected, Browse... button while a receive is running, etc.)
• Support drag and drop
• Override default menus
• Support "undo" and "redo" in text fields
• Support transferring whole directories. However, there may be security implications associated with this (e.g., what if someone tries to transfer the root directory?).
• Make maximum bitrate and redundancy configurable
• Check only one network interface uses the 10.0.1.255 range: otherwise, data could be send through a network interface that isn't connected to the data diode