WindowServer: Show PID and window ID in title bars for now.

This is useful for debugging since I'm often wondering which process some
window belongs to (and what the window ID is.)

WindowServer/WSWindow.h

@@ -38,6 +38,8 @@ public:
 
     GraphicsBitmap* backing() { return m_backing.ptr(); }
 
+    pid_t pid() const { return m_pid; }
+
     // For InlineLinkedList.
     // FIXME: Maybe make a ListHashSet and then WSWindowManager can just use that.
     WSWindow* m_next { nullptr };
@@ -51,5 +53,6 @@ private:
     RetainPtr<GraphicsBitmap> m_backing;
     Process& m_process;
     int m_window_id { -1 };
+    pid_t m_pid { -1 };
 };