Add animated gif dropdown to level editor

1. Remove unused `properties.instructions_2` level attribute
1. Add animated gif drop-down field

![image](https://cloud.githubusercontent.com/assets/206973/3333905/99d5dda0-f7f8-11e3-9f19-4516fa772294.png)

![image](https://cloud.githubusercontent.com/assets/206973/3333907/a0beda7c-f7f8-11e3-930f-353195f36c27.png)