Centralize filter form processing via filterform helper

• Add filterform helper
• Move filter form processing from the ruleform helper to the filterform helper.
  • (Move ruleform to the helper namespace)
• Move filter form processing from the filter_manager to the filterform helper.
• Adjust dataformfilterform standard and advanced to use the centralized form definitions.

Filter form settings changes

• Change per page input element to text box and add validation.
• Change number of additional criterion slots to 2.
• Add Continue button in the advanced filter form.

Filter form behavior changes

• Redirect advanced filter form to the filtered view on save.

Internal API

• Allow passing a filter to view display (via the options argument).

Entries filtering

• Move separate groups mode filtering from \access\base (back) to entry_manager::get_sql_query

Additional Filter attributes

• states - a list of states to fetch entries in particular states.
• grouped - override Dataform grouped setting.
• individualized - overried Dataform individualized setting.
• groupmode - override Dataform group mode.
• currentgroup - override Dataform current group.