Allow GIFT imports to set the general feedback for questions

This would be useful, we just need to find a way to implement this which is mostly backwards compatible.

The suggestion is to have a new separator, like ## to separate the general feedback from the rest of the question definition.

The danger is that people are already using the character sequence ## in their questions for other reasons, in which case this change would break their import files.

So, it is a trade-off. Useful new feature, at the risk of breaking existing GIFT files. We can reduce the risk by using a more unlikely character sequence like #### or #~#. Really, someone needs to make a design decision.