(fix) WCoverArt: popup on click release

[ronso0]

Fixes #13181
Forgot to backport #13035

Though, I'm confused why this can break so easily.

FWIW, this is what various button tests return for trackpad click release with Qt 5.12.8:
FALSE pEvent->buttons() & Qt::LeftButton
FALSE pEvent->buttons() == Qt::LeftButton
FALSE pEvent->buttons().testFlag(Qt::LeftButton)
TRUE pEvent->button() == Qt::LeftButton (fix)

• various widgets' mousePressEvent() succesfully use
  pEvent->buttons() == Qt::LeftButton
• in release events however we use
  pEvent->button()

[ronso0]

FWIW I discovered this by chance when trying to view a cover in a live session.
We should set up a smoke test check list. Or,if such an official list exists, add some cover test cases.

[daschuer]

I can confirm the issue is fixed. Thank you.