Refactor Global Spacebar Play (#7926)

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.

include/Editor.h

@@ -51,6 +51,7 @@ class Editor : public QMainWindow
 public:
 	void setPauseIcon(bool displayPauseIcon=true);
 	QAction *playAction() const;
+	static Editor* lastPlayedEditor() { return s_lastPlayedEditor; }
 protected:
 	DropToolBar * addDropToolBarToTop(QString const & windowTitle);
 	DropToolBar * addDropToolBar(Qt::ToolBarArea whereToAdd, QString const & windowTitle);
@@ -73,6 +74,8 @@ protected slots:
 
 private slots:
 	void toggleMaximize();
+private:
+	inline static Editor* s_lastPlayedEditor = nullptr;
 
 signals:
 

include/Song.h

@@ -254,7 +254,6 @@ public:
 		return m_playMode;
 	}
 
-	PlayMode lastPlayMode() const { return m_lastPlayMode; }
 	inline PlayPos & getPlayPos( PlayMode pm )
 	{
 		return m_playPos[static_cast<std::size_t>(pm)];
@@ -490,7 +489,6 @@ private:
 	std::array<Timeline, PlayModeCount> m_timelines;
 
 	PlayMode m_playMode;
-	PlayMode m_lastPlayMode;
 	PlayPos m_playPos[PlayModeCount];
 	bar_t m_length;