LibGUI: Only dispatch Leave if the now-hovered widget isn't a child

Without this change, when the mouse starts hovering over a child widget,
the parent would receive a Leave event despite the parent widget not
losing mouse hover.

Userland/Libraries/LibGUI/Widget.h

@@ -280,6 +280,8 @@ public:
 
     bool has_pending_drop() const;
 
+    bool is_parent_of(Widget const*) const;
+
 protected:
     Widget();