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

Make access plugins a little less focused on the term "access" and change selection criteria to selection rules.

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. 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.