Fix chapter div width

When `#editorPane` is displayed then the half of `#chapter` element was overlapped under `#editorPane` because of `width=100%`.

Set its width to absolute value 600 (canvas width) + 2 + 2 (canvas left and right paddings) = 604px.
Absolute values are acceptable here due to `canvas` size is also absolute.

![wrong-chapter-div-width](https://cloud.githubusercontent.com/assets/598919/4827694/771736da-5f7a-11e4-943d-ac141f335f8f.png)