Add a Cancel button

[ryandesign]

I use youtube-dl on the command line, but I installed Virga for a less computer-savvy relative. The first thing she did after I added the Virga app icon to her Dock was to click the Virga icon multiple times, which caused it to launch multiple times, and to display the "video or audio" selection dialog box multiple times. There was no Cancel button, so we could not cancel the erroneously-started downloads.