symbology: unique value/categorized -> allow data driven labels

[lutra]

it would be really useful to allow users define labels using the values of a column in the table of attributes.

It would be also useful to allow data driven symbol color RGB values.

[mhugent]

I'm currently working on data dependent labelling-ng (data dependent RGB values is something different. Probably it should be a ticket on its own)

[mhugent]

The attached patch adds support for data defined font and buffer settings to the new labeling. Martin, could you review it?

Thanks, Marco

[wonder]

Marco, I've checked the patch and it looks fine.

I would also remove the legacy prepareLayer() call as I don't think we need to care about backwards compatibility here (labeling-ng is the only user of that interface). Additionally, I would use QList<in> instead of QSet<int> for the list of attribute indexes - a list is used also when passing the indexes to layer/provider.

[mhugent]

Applied in r13998. Thanks for reviewing, Martin. I removed the legacy method but kept the QSet (it is a bit easier for the label engine to filter duplicated fields).