Work Progress - Dec 8 - Dec 14

Demo: here
Google doc TODO list: here

Done:
* Fix the bug that the variable name changed when moved and saved in the scriptlet window.
* Avoid invalid coordinate input by having users enter the coordinates in a pop-up window. (Try editing coordinates from the property window)
* Assignment, binary statements can now drag and drop freely throughout the application. Download and load this XML for an illustration of what can be done by drag-and-drop.
* Can now control whether the skateboarder yells something during simulation (with its "Start Yelling()" and "Stop Yelling()" method codelets, and what to say by its "Change message to yell to()" method codelet)

Mayan's suggestions
* [Done] Make grid the default, show # as well
* [Done] turn parameters to integer if possible
* [I move them to the advanced properties] hide the gravity parameters
* [Use tooltip text to explain the meaning of a negative value] name of the acceleration parameter
* [Fixed] omega in property dropdown