Fixed|Mobile|libappfw: Mutex misuse in desktop build

In the desktop build, the main thread does all the UI processing,
including drawing. The root mutex is intended to prevent the render
thread from accessing widgets while their state is changing.

doomsday/sdk/libappfw/src/widgets/dialogwidget.cpp

@@ -581,24 +581,30 @@ int DialogWidget::exec(GuiRootWidget &root)
     root.add(this);
 
     prepare();
-    
+
     int result = 0;
     try
     {
+#if defined (DENG_MOBILE)
         // The subloop will likely access the root independently.
         root.unlock();
-
+#endif
+
         result = d->subloop.exec();
-
+
+#if defined (DENG_MOBILE)
         root.lock();
+#endif
     }
     catch (...)
     {
+#if defined (DENG_MOBILE)
         // The lock needs to be reacquired in any case.
         root.lock();
+#endif
         throw;
     }
-    
+
     finish(result);
     return result;
 }