#863298 by donquixote, yhahn: Only reset the block info cache when a form is submitted in the admin UI.

Introduce hook_context_node_condition_alter() to allow other modules to easily add their plugin to node-related events.

Ensure block info cache is not stale after administrative actions alter block info (e.g. creating new views)

Reduce the number of calls to drupal_alter() - move alter from property fetch event to context_load().