Add Linux support, scrollbars, clipboard, better exit shutdown support

[rdragonrydr]

I got a little carried away, this isn't just code to add Linux support (start/stop ffmpeg, remove the early exit checks) but I also added:

• Ability to click the list of processed videos to copy the file path to clipboard
• Horizontal and vertical scrollbars for both pre- and post-process video lists
• Font change for video list to discern visually-similar characters better
• Logging change to also print to terminal (admittedly something of a hack)
• Exit button now automatically opens the "kill ffmpeg" window if running, otherwise both it AND the kill-ffmpeg button simply exit the program without extra dialogs
• ...Probably a few minor things I forgot

resolves #24

[JuanPabloPearce]

@nhershy