FormFieldsTagLib private to protected accessors

For futher extensiility of TagLib the private accessors has been replaced with protected.

Example of custom extensiblity:
```
package com.selly.taglib

class OverrideFieldsPluginTagLib {

    static namespace = "f"

    def field = { attrs, body ->
	// extra validation attributes goes here, but
	// grailsApplication.getDomainClass(beanClass.name) is needed
		
    	def fieldsTagLib = grailsApplication.mainContext.getBean('grails.plugin.formfields.FormFieldsTagLib')
        fieldsTagLib.field.call(attrs, body)
    }
}

```