Work Progress - Feb 23 - Mar 1

* Users can now drag a codelet even if we have a textfield underneath it in the scriptlet panel. (ref)
* Fix the bug that the background object appears twice in the applet version at the beginning.
* The representation of object properties has been changed: a label for each property instead of a pull-down menu.
* Textfield for number/string input is now the default entry for the method codelet, assignment codelet, etc. User can no longer pull the textfield away. Such a behavior is similar to what we have in MIT Scratch.

TODO:
* We should record the layers where each event codelet is currently located in the scriptlet panel. Otherwise we may not be able to reproduce the exact original layout when we reload the scriplet panel.