Patch - data overlay views plugin should use field handler UI names

In some cases, latitudes and longitudes come from the same field, similarly named fields, or inaccurately named fields. In the Views UI, site builders can configure the field label or an "administrative title". The data overlay style configuration form should use handlers' "UI name" rather than the name from the field definition.

Before: https://img.skitch.com/20111008-d12jnb9bsr6j98m6d2ge7qw7pd.png

After: https://img.skitch.com/20111008-rf1qe9jyd2we4t4kdiykwkcais.png

The handler ui_name() method will use the the administrative title, then the field label, and ultimately will fall back on the name from the field definition.

Extremely simple patch attached.