Songs-to-youtube

- Added some new functionality to template strings (see https://github.com/7x11x13/songs-to-youtube#operators)
- Updated settings version to v0.8 with default settings utilizing the new functionality

- Made logging while uploading thread-safe, no longer crashes when loglevel is debug and doesn't mess up the log window
- Removed selenium_firefox dependency, now just relies on selenium

- Program now searches for cover art files when songs are added, and there is now a setting "Prefer cover art file over embedded art." This is enabled by default because cover art files are usually higher quality than the embedded art.
- Added a new command "no background" which renders videos to be the exact dimensions as the cover art supplied. In browser, youtube will fill in the background with black, but on youtube mobile the video will have no background.

- Google doesn't let you login to YouTube while your browser is automated anymore so you have to manually save browser cookies yourself. Added instructions for how to do this under File > Settings > Add new user
- Slightly more informative selenium errors
- Fix uploading when user has no playlists or very few playlists

- Changed the way the settings are stored again so it'll reset them on launch (sorry). The ffmpeg command strings are now each stored in their own file in `./commands/render` and `./commands/concat` instead of in the Qt settings file.
- The default render commands now render anything pretty much instantaneously, however the videos take longer to be processed on YouTube
- Added a button to cancel while rendering/uploading