Enable horizontal direction of manipulation

Extend `FloatModelEditorBase` so that it also allows manipulation of the
values when the mouse is moved in horizontal directions. The default is
to use the vertical direction.

Make use of this new feature in `BarModelEditor` which now reacts to
horizontal movements when changing values.

include/FloatModelEditorBase.h

@@ -52,7 +52,14 @@ class LMMS_EXPORT FloatModelEditorBase : public QWidget, public FloatModelView
 	void initUi( const QString & _name ); //!< to be called by ctors
 
 public:
-	FloatModelEditorBase( QWidget * _parent = nullptr, const QString & _name = QString() ); //!< default ctor
+
+	enum class DirectionOfManipulation
+	{
+		Vertical,
+		Horizontal
+	};
+
+	FloatModelEditorBase( DirectionOfManipulation directionOfManipulation = DirectionOfManipulation::Vertical, QWidget * _parent = nullptr, const QString & _name = QString() ); //!< default ctor
 	FloatModelEditorBase( const FloatModelEditorBase& other ) = delete;
 
 	// TODO: remove
@@ -109,6 +116,8 @@ private:
 	QPoint m_lastMousePos; //!< mouse position in last mouseMoveEvent
 	float m_leftOver;
 	bool m_buttonPressed;
+
+	DirectionOfManipulation m_directionOfManipulation;
 };