Commit 0b2b243b removed the call to KGlobalAccel::setGlobalShortcut() because it registered Space as global shortcut, which of course is a bad idea.
But that made it impossible to set up a global shortcut at all (and made an existing one have no effect).

To fix this, add it back and register an empty key, like the KDE4 code does.

A global shortcut for Play/Pause can again be set in systemsettings5->Shortcuts->Global Shortcuts (it doesn't have a key assigned by default), and pressing the key does work as expected afterwards even if Amarok's window is not active.

The local shortcut (Space by default) still works as well (but only if Amarok's window is the active one, as it should be).