LibWeb: Sort stacking context tree once, after fully building it

Instead of calling quick_sort() every time a StackingContext child
is added to a parent, we now do a single pass of sorting work after the
full StackingContext tree has been built.

Before this change, the quick_sort() was ~13.5% of the profile while
hovering links on GitHub in the Browser. After the change, it's down to
~0.6%. Pretty good! :^)

Userland/Libraries/LibWeb/Painting/StackingContext.h

@@ -33,6 +33,8 @@ public:
 
     void dump(int indent = 0) const;
 
+    void sort();
+
 private:
     Layout::Box& m_box;
     StackingContext* const m_parent { nullptr };