Support individual theme generation by values in a "lookup" Class Definition

[jng]

When generating a theme of individual values, the actual values are being sampled from the Class Definition the layer will render off of.

If the property being themed on is a foreign key to some external lookup table, the theme that gets generated isn't very meaningful as any generated labels will be using the "ID" value instead of the "label/description" value that would normally be in the lookup table that the foreign key links to.

In such a situation, it would be very useful to offer the user the ability to generate a theme based on values of the lookup table itself.

[jng]

Fixed r7991 and r7992