refactor Fields / #2080

Problems:

• double usage of field-lists:
  • fields - assoc. array of field_name => Field object (reference to global array like $g_task_fields)
  • field_states - assoc array of field_name => status actually meant to lazy loading from database. Now only used for checking if a Item has a field
• global variables like $g_task_fields
• weird initialization of fields (must be init outside of Class definiation because of translation issues)
• Field-Definitions are not commonly available in static functions like Task::query() which makes them harder to be used for building SQL-queries
• rendering and parsing of Forms is current obscur (scattered over db_item.inc.php, render_fields.inc.php and render_form.inc.php).

Changes:

• completely remove DbItem::field_states
• add static function DbItem::init() as factory method after class definition (e.g. just behind Class Task { ... } )
  • move field definition into this function
• make DbItem::fields a static member
• assemble all field-related functions to new sourcefile: db/db_fields.inc.php

• refactor Filters