Make lyrics editable and implement context menu for copying

Hello! Thank you for fixing #17656 (closed)! But there are some more things for developing.

1. When I want to copy lyrics, I make right-click for copying, but there is no context menu. Ctrl+C works for it, but it's not intuitive, and many people may not to guess about it.

2. It would be good to make it editable as General metadata.

I've tested it on http://nightlies.videolan.org/build/win64/last/.