Add a layer list to the Identify Super Tab

[brentfraser]

Currently there is no way for the user why the Identify did not show results. If no results are returned it could be because:

• the layer is not turned on (visible)
• the layer has not been configured as identifiable by the administrator
• the data server has not responded (at least as GM expects)
• the user's query geometry did not result in any results

A layer list would give some immediate feed back to the user on the first two reasons (and they can do something about the first one).