WindowServer+LibGUI+Taskbar: Store window progress as Optional<int>

We were previously using the magical constant -1 to signify that a
window had no progress state. Be more explicit an use an Optional. :^)

Userland/Libraries/LibGUI/Window.h

@@ -190,7 +190,7 @@ public:
 
     Window* find_parent_window();
 
-    void set_progress(int);
+    void set_progress(Optional<int>);
 
     void update_cursor(Badge<Widget>) { update_cursor(); }