Fix chapter div width

When `#editorPane` is displayed then 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 now due to `canvas` size is also absolute.
Merge pull request #303 from phts/fix-chapter-div-width

Fix chapter div width