WindowServer: Move the CPU usage graph updates to a secondary thread.

This avoids blocking the main thread on filesystem access, which created
noticeable stutters during compilation.

AK/CircularQueue.h

@@ -70,6 +70,8 @@ public:
     ConstIterator begin() const { return ConstIterator(*this, m_head); }
     ConstIterator end() const { return ConstIterator(*this, size()); }
 
+    int head_index() const { return m_head; }
+
 private:
     friend class ConstIterator;
     T m_elements[Capacity];