#98 Extend attribute list scale generator

[Peter Becker]

The attribute list scale generator and the nominal one
are doing more or less the same, with the attribute
scale generator more flexible and the nominal one more
easy to use. They should somehow be combined, with the
nominal scale generator being a supplier for the
attribute lists.

This could even be extended to the ordinal and
biordinal scales, which means they would all be
suppliers for the attribute list view, actually making
the biordinal dialog superflous, since it can be
replaced with two calls to the ordinal one.

If we then add a feature to extract the attributes from
any context/diagram, both context and attribute list
would be extra views on the formal context, in addition
to the diagram. This could greatly enhance the UI in total.