Refactor Global Spacebar Play

[regulus79]

This PR simplifies #7762 by removing the editor-specific code from MainWindow and m_lastPlayMode from Song, and instead uses a static variable in Editor which keeps track of the last played editor.

This PR also removes the spacebar shortcut from MixerChannelView for changing the volume, as multiple users have noticed that it clashes with their intuition after getting used to the spacebar working everywhere else.

[sakertooth]

Could we also undo the error-prone event propagation as well? Each widget can have their own keyPressEvent, and each decide what it should do when the spacebar is pressed. Most widgets can then just decide to play the editor.

[sakertooth]

Could we also undo the error-prone event propagation as well? Each widget can have their own keyPressEvent, and each decide what it should do when the spacebar is pressed. Most widgets can then just decide to play the editor.

Actually, maybe its fine, I just want to prevent bugs like #7915 from happening.

[bratpeki]

Ready for testing?

[regulus79]

Yes! The only visible change should be that the mixer channels don't respond to the spacebar anymore.