Warn if colour scheme cannot be exported

[scottclowe]

The code really needs to give a warning if the user has not visited all the preference panels.

Currently, the problem is a value is returned by the java getPrefs function even if the value is not available - it is just the wrong value is returned. But I can check if all the values from a panel are returned as black -16777216, and if so assume the panel is not being exported correctly.