BackendModel::getExtrasForData, added $action

You can now narrow your search down with $action.
BackendModel::deleteExtrasForData, added $action
Only invalidate frontend cache when necessary
Updated changelog
Minimalized the query
Fixed typo
BackendModel:: deleteExtrasForData needed to be static
Core: add BackendModel::getExtrasForData + deleteExtrasForData.
Core: add BackendModel::getExtrasForData + deleteExtrasForData.