form.field.Select: picker content styles

@maxrahder @Dinkh: The picker DOM did live inside the form field DOM node when it initially got implemented. This had some issues, in case a field lived inside a container with overflow: hidden. The new implementation lives directly inside document.body.

However, I did notice that some style rules are still living inside the .neo-selectfield rule and we should double-check if they still get applied as they should.

In general: a picker field should probably pass its on baseCls to the picker container, unless this is already in there.