WindowServer: Compute final window title before passing to WM clients

We were not substituting the window modified marker ("[*]") in the
title strings we were sending to WM clients. This caused the Taskbar
to show pre-substitution window titles for the Text Editor application.

This patch moves the window title resolution to Window::compute_title()
which is then used throughout.

Userland/Services/WindowServer/Window.h

@@ -125,6 +125,8 @@ public:
     String title() const { return m_title; }
     void set_title(const String&);
 
+    String computed_title() const;
+
     float opacity() const { return m_opacity; }
     void set_opacity(float);