Don't display 'Help' action in context menus when there's no help text

Instead display a disabled item saying "Help (not available)".
![screenshot - 141114 - 15 47 19](https://cloud.githubusercontent.com/assets/2879917/5047417/90dd6bdc-6c15-11e4-8c6d-7e41d5b65631.png)


Fixes #1290