Expose trackUsage flag in page provider (and content view) definition

This flag is needed to be able to have statistics about the searches: Admin > Activity > Search Analytics

TODO

• Expose a new option in the Page Providers and Content Views modeler screens.
• Where: in the advanced configuration zone, in the bottom
• Conditions: active only when ES is used for the pp / cv, disabled otherwise
• Type: boolean
• Default: true
• Label: Generate Usage Statistics
• Contextual Help - ( ? ) button: Only available when query makes use of Elasticsearch. Activate this option to gather usage statistics on the usage made of this feature. You will be able to consult them in the admin center's analytics view and to leverage them in your own elements.

HANDLING THE EXISTING
Option should be true:

• for new content views / pp when making use of ES

Option should be false:

• for existing content views (meaning customers will have to activate it manually for their existing content views / pp)

Option should remain as is:

• for existing content views, when "use ES" option is enabled / disabled / reenabled

TARGET PLATFORMS
7.10+

XML GENERATION
This option should add the trackUsage flag to the generated page provider (when ES is used for the page provider)
e.g.:

<coreQueryPageProvider name="Whatever">
<trackUsage>true</trackUsage>
[...]

http://explorer.nuxeo.com/nuxeo/site/distribution/Nuxeo%20Platform%20LTS%202016-8.10/viewExtensionPoint/org.nuxeo.ecm.platform.query.api.PageProviderService--providers