Allow restricting contexts via access control to allow the add content UI to remove irrelevant content.

Expand the context to allow the plumbing for replacing placeholder contexts with real contexts for preview.

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.

Checkpoint checkin. Lots more interesting functionality -- pages can now be set to "single" or "multiple" task handlers, and having "single" removes the task handler arrange UI. Please some serious code re-organization so that Panels can have just one context-based task handler for multiple tasks.

Checkpoint checkin: Can now create arbitrary pages! Yay. Task handler manipulation on arbitrary pages works, though there are some clear disconnects in the UI that will be addressed next. Not all page admin functions exist yet (no clone, enable, disable, revert, delete). Not all arguments have criteria yet.

Checkpoint checkin. Partial implementation of page task with generic subtasks, plus revamp of arguments, new access plugins (stolen from Views), updates to the wizard to make it fully compatible with the modal and some minor improvements to make it more flexible.

- 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.