Implement context menu for copying in metadata panel

Hello! Thank you for fixing #17656 (closed)!

But there is a problem: 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.

I've tested it on http://nightlies.videolan.org/build/win64/last/ build for 11.03.2017.

Edited Apr 04, 2021 by Jean-Baptiste Kempf

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