Adding proportional scrolling (#7476)

Add proportional scrolling to the song editor, piano roll and automation editor. Proportional scrolling means that if for example a certain measure is on the right side of the song editor then it will take a certain number of mouse wheel moves to get it to the left side of the editor. It is the same number of wheel moves regardless of the zoom level.

include/AutomationEditor.h

@@ -241,6 +241,8 @@ private:
 	QScrollBar * m_leftRightScroll;
 	QScrollBar * m_topBottomScroll;
 
+	void adjustLeftRightScoll(int value);
+
 	TimePos m_currentPosition;
 
 	Action m_action;

include/PianoRoll.h

@@ -374,6 +374,8 @@ private:
 	QScrollBar * m_leftRightScroll;
 	QScrollBar * m_topBottomScroll;
 
+	void adjustLeftRightScoll(int value);
+
 	TimePos m_currentPosition;
 	bool m_recording;
 	bool m_doAutoQuantization{false};

include/SongEditor.h

@@ -128,6 +128,8 @@ private:
 
 	QScrollBar * m_leftRightScroll;
 
+	void adjustLeftRightScoll(int value);
+
 	LcdSpinBox * m_tempoSpinBox;
 
 	TimeLineWidget * m_timeLine;