Drop "label field"

As pointed out in #60 a single "value field" that contains a string suitable for display/preview purposes is not going to work. The most simple example is `LatLongValue`. We store it in two decimal columns. Everything else would be odd. Both columns are needed to create a label. The code to create labels from these columns must be in the handler.

What the handlers need is:

1. An optional `getDataValue` method that either returns `null` by default or a `DataValue` if it's possible to reconstruct it from the table.
2. An optional `getLabel` method (or `getPreviewLabel` or something like that) that returns a string that's suitable for preview purposes (but not necessarily an exact representation of the original `DataValue`).