Add is k1 level attribute

Adds a Level.is_k1 attribute and exposes it to the generic level and maze editors.

![image](https://cloud.githubusercontent.com/assets/206973/2884930/8bb10d32-d4b5-11e3-9fdc-24f64e699831.png)

![image](https://cloud.githubusercontent.com/assets/206973/2884931/9ac87a30-d4b5-11e3-9445-4f0f4b3ff196.png)