Implement context keywords which also use the converters. Add converters to user, fix taxonomy converters.

Retool the access plugins to allow multiple plugins plus and/or switch. Retool selection criteria to use this allowing for a much richer selection of task handlers on a given page.

- a tool for AJAX, which is the next generation tool based on what Views uses.
- a tool for modals -- completely ripping the modal out of Panels.
- moving plugins for 'arguments', 'relationships' and 'contexts' into CTools.