Initial checkin of an attempt to abstract the basic ideas of the page manager UI into something anyone can use in the CTools export UI. Currently missing CSS and documentation.

This reverts commit 1704f695.

Issue #1168272: Retool block.inc to support block_list_alter and thus things like i18n block translation.

Issue #1235218: Restore "node" keyword to node add/edit forms so they can act like node contexts again.

Issue #1223532: Multiple terms required context not available to Views and Multiple term required context not usable as a single term context because of entity change over.

Issue #1195686: Typo in entity name causing notices and broken context on term from node relationship.

Issue #1193886: node edit form often only has "Form goes here" due to bad recursion prevention code.

Issue #1043606 by Les Lim: Allow stored custom content links to have an admin link to configure them.

Issue #1139918 by entrigan: Work around brokenness with node edit form context by not treating it as a pure node.