Bring the entityreference selection handler up to date

This is the future proof solution, which creates a separate directory for the plugins.
However, it might cause fatals when the class moves, requiring the use of http://drupal.org/project/registry_rebuild to fix it.

The other solution is to just ignore 'behavior' plugin types, and not move the class. It will hurt us in the long run if we want to add behavior plugins.

Making this more generic.
There were a bunch of changes made to the selection plugin that we can build upon, or have to adapt to.

Also, while we're currently using self::TYPE in some places to ensure that this is only used for webform_entity, this patch cuts that down to a minimum to make it easier to genericize later.

Committed, thanks!