Index plugin data related to pages but stored elsewhere, e.g. tags in DB

[annda]

This PR provides new events that allow plugins to save additional page-related data into Elasticsearch index:

• PLUGIN_ELASTICSEARCH_CREATEMAPPING gathers additional index field definitions
• PLUGIN_ELASTICSEARCH_INDEXPAGE gathers the values of those additional fields

In addition, the public helper method \helper_plugin_elasticsearch_plugins::updateRefreshState lets plugins inform elastic that re-indexing is due, even if the page content itself has not changed.

Two more events let plugins integrate into the search itself:

• PLUGIN_ELASTICSEARCH_FILTERS will add a checkbox filter to advanced search tools, based on provided configuration
• PLUGIN_ELASTICSEARCH_QUERY lets plugins manipulate the query string, as well as provide some additions that should be appended, e.g. #foo when option foo is selected in the plugin's search filter

[annda]

Looks good. It's still missing the list of fields the SimpleQuery should search in (and the hook for plugins to extend it), right?

This was added in 0c7a4d0