UI: Do not remove focus on mouse leave events

For the "ignore wheel" widgets, do not leave focus on mouse leave
events.  This causes unintended side effects such as not being able to
type in edit boxes, and is very annoying.

UI/slider-ignorewheel.hpp

@@ -17,7 +17,6 @@ public:
 protected:
 
 	virtual void wheelEvent(QWheelEvent *event) override;
-	virtual void leaveEvent(QEvent *event) override;
 };