Fix #5851: Implement EffectRackView::sizeHint() (#7428)

* Fix #5851: Implement `EffectRackView::sizeHint()`

This fixes `EffectRackView` to have a permanent size hint instead of
resizing the widget once in `InstrumentTrackWindow`. The size hint tells
the `InstrumentTrackWindow` to not increase with a growing number of
effects in the `EffectRackView`.

include/EffectRackView.h

@@ -64,6 +64,8 @@ private slots:
 
 private:
 	void modelChanged() override;
+	QSize sizeHint() const override;
+	QSize minimumSizeHint() const override { return sizeHint(); }
 
 	inline EffectChain* fxChain()
 	{