Songs-to-youtube

- If browser is launched in headless mode and an error occurs while uploading, take a screenshot of the browser and save it
- If the browser is not in headless mode, don't close the browser if an error occurs while uploading

Potential fix for 7

- Add headless browser mode, enabled by default
- Show progress bar while uploading, also improve logging while uploading
- Actually quit browser when cancel button is pressed

Use QThreadPool for rendering, hopefully improving stability

Bugfixes, v0.8.0 cannot render albums as single videos

- 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